Eriadu sounds like a good option, due to its history with the Tarkins. so if you're looking at four big movements and then done, I'm not sure this is going to mesh with that expectation. My point in regards to that comment was to try to explain my perspective. While this event may go on for a while and many writers here will fill up that 24 episode season, I can't.
Strategically the Ssi-Ruuk would have very little interest in Bespin, using a small task force at most to entech Cloud City and then maybe park a Lwhekk manufacturing ship or two in orbit to harvest Tibanna for their war effort, but it would be a very low priority for immediate invasion, kind of like island hopping, something that could be bypassed for now and then come back to it later. A potential way to involve Bespin indirectly in the beginning would be to have it become a staging point for the local Moff's sector fleet when it goes out to battle the Ssi-Ruuk. A half dozen Imperial Star Destroyers, the Moff's personal Star Dreadnought, and forty or so other ships assemble then head out. A few days later they see the survivors of the Imperial's phyrric victory; A couple of Imperial Star Destroyers that are battered and damaged, and maybe half of the forty support ships. No Star Dreadnought and that would draw the attention and talk of people on Bespin. The local Sector Fleet stopped the Ruu push temporarily at the cost of expending its strength and being unable to stop them again without reinforcements from the Core region Imperial worlds.
Looking at the Atlas, I believe Manpha (Imperial Space) and Rutan (Alliance Space) would be the best candidates for the beachhead worlds. Manpha is the next significant target on the Corellian Trade Spine and is about halfway between Terminus and Bespin. Rutan is the next significant world on the Hydian Way and is very close to Mustafar, which is home to an important Alliance shipyard and has decently significant Cizerack mining facilities. Behind those two worlds and surrounding Terminus is a host of more minor planets that could be gobbled up in a hurry by the Ruu as they expand their territory while working to Entech Terminus and bring in their Lwhekk manufacturing ships to start replacing lost equipment and weapons. These worlds are all in Imperial Space and include; Saijo Anoth Kinooine Delrakkin Subterrel Polis Massa
Polis Massa and Subterrel are both in Alliance space, as clearly marked on the map. The Moff for the Greater Javin (Ceto Rübezahl) already stages out of Bespin - as far as I am aware, I do not have a Star Dreadnaught: I think under old board rules from ye olde fleeting days, they were restricted to Grand Admirals only to avoid us having an absurd number of SSDs on the boards. If someone is going to be leading the Imperial offensive at Eriadu though, it would probably be the Grand Moff for the region (Augustus Valorum). Unless you're meaning the Eriadu Moff, who presumably would be at Eriadu when the Ssi-Ruuk first show up? Hitting worlds along those routes is a good option for flavour... but if we're taking such a huge chunk out of the Alliance's mining operations, we're definitely going a bit against the grain on the whole "the Empire is going to suffer the brunt of this" situation - the Alliance is suffering far worse than the Empire at this point, and with the Empire getting Chiss help as well? I guess the question is, are the Ssi-Ruuk actually interested in depriving the Alliance of material resources? Do they even care about the Alliance at this point? Like Maggewetok pointed out earlier, their aims and objectives here are probably an important thing to figure out.
I must confess, I was looking at the Atlas in my guide and trying to mentally remember where the border was. I was also thinking in general Imperial terms and not 'Fans Imperials, so yeah that makes sense about the Star Dreadnoughts, although there are different ones ranging in size and capabilities; While Executors and Assertors are 19km and 15km in size and fearsome in strength, Bellators are only 7.2km in length and are a scaled down version of the earlier Mandators. But if that's still too much then perhaps a Star Battlecruiser, such as a Procurator? I wasn't hinting at an attack on Mustafar, only that it would be threatened by the beachhead into Alliance space. For now I would say the Ruuk only push a little into Alliance space, mainly going after Refugees, but the Corellian Trade Spine offers a more lucrative route into the Core as far as mass entechments go to power their war fleet. I could see an occasional raid for raw resources from the mining planets (only because it would cut down on the amount of work needed for their own mining) but perhaps one or two early defeats at their hands by the Alliance would caution them to avoid them for now and go after the easier Imperial prey? Certainly the Alliance would fight a lot harder on behalf of the Refugees than the Imperials would and this may instill caution in the Ruuk for going into Alliance space for a while.
The plan with Cloud City and the Greater Javin is/was to have much of the security and enforcement addressed by a private group, rather than the Imperial military. The thinking is that with the various occupations and planetary bombardments that the people of the Greater Javin have endured over the decades, they would not be receptive to an overt military presence, and since the Greater Javin seeks to replace the now defunct Corporate Sector, presumably corporations will find a non-military feel a bit more welcoming. From a narrative perspective too, it is a bit more exploitable for Cloud City's numerous criminal groups. Unfortunately I've had trouble finding other writers to help bring that into play gradually, but if we're jumping to 12 ABY I suppose there's a little wiggle room to hit the fast forward and say that the private forces are in place when this goes down. Obviously, protecting the Greater Javin from the Ssi-Ruuk with (not exclusively, but mostly) private security is a bit untenable, so I'm guessing the Empire might bolster whatever military forces the region still retains for defense. If that includes a new flagship for Moff Rübezahl that is of Star Destroyer size or larger, him grudgingly making use of that is something I think I can work with. (I'm having thoughts about doing an Isde Naha thread anyway, if it doesn't take anyone else's fancy) On the topic of Super Star Destroyers though, it could be very interesting to have one of those at Eriadu. The epitome of Imperial engineering being destroyed by the Ruuk could be our USS Odyssey moment (to steal a DS9 analogy again) - something iconic and symbolic of the brand being taken out by this new aggressor to help underscore how high the stakes are. Based on Moff Tarkin's example, I'm assuming my Grand Moff would not command something so large... but perhaps Tear (as Executor) could ride that particular warhorse into battle?
I guess the question is, are the Ssi-Ruuk actually interested in depriving the Alliance of material resources? Do they even care about the Alliance at this point? Like Maggewetok pointed out earlier, their aims and objectives here are probably an important thing to figure out. I think these are an important series of questions. Maggewetok does bring up a good point. What are the Ssi-Ruuk's aims and objectives? Are they arrogant enough to believe themselves so superior, above non-saurians, that opening a two front war is an acceptable strategy? We could make WW2 parallels with this invasion. Making the Ssi-Ruuk (Axis), The Empire (the allies), The Chiss (the states), and the Rebels (Russia). Obviously, this metaphor is only for chronological events not characterization. The Empire is put on the ropes early and left for dead. The Ssi-Ruuk, opens a second front against the Rebels. The Chiss join the Empire and they rejoin the war. And together we push toward Berlin, or Terminus. This being a very rough outline of possibly chain of events. We would have to think all the inbetween stuff. Like how the Ssi-Ruuk would begin to crumple and how much they might chew up the Rebellion before the Empire gets back on their feet. Would the Ssi-Ruuk pretend to be friends in the same way Germany was to Russia early on? Would the Ssi-Ruuk actually parlay with the Rebellion to any degree? Or are they so xenophobic would they balk at the mere idea of such a thing? I'm not terribly familair with them lore wise and I think a lot of this depends on how intelligent we want to play them. but perhaps Tear (as Executor) could ride that particular warhorse into battle? I guess we'll see. I was rather hoping Miranda as Empress would lead the charge toward Eriadu.
Even more fundamental than that... why have they attacked at all? If the Ssi-Ruuk are just after resources (including slaves, etc), they might be looking at Terminus as a beehive. They want the honey that's inside, but in order to get it you have to be prepared to get stung by a few bees. They'll likely have brought enough ships and soldiers to keep the Imperial bees at bay, but - bearing in mind that the Empire still spans a significant chunk of the galaxy, and still has a sizeable military - not necessarily to wipe out the Empire completely without breaking a sweat. That might mean that the Ssi-Ruuk advance is slow and mythodical, moving through the Empire like a plague of locusts - harvest everything you can from one planet, then move onto the next, and the next - and that there'll be time for the Empire to gather intel, formulate a plan, and get their shit together before the Battle of Eriadu goes down. That might also be a good justification for why the Empire bears the brunt of it at first: if they're after honey-rich worlds full of people and technology, they're going to want to stear towards the Core and the Colonies, not the relatively backwater worlds that make up the bulk of the Alliance. Also, humans are "better" as a source of entechment energy than some races: perhaps that's another reason for the Ruuk's bias towards the human-rich Empire over the alien-rich Alliance. Something to bear in mind there, though: the Empire used to have enough military might to keep the entire galaxy in line, and while they've lost some of that, they've also halved the size of their territory, so they're not exactly short on ships. The Empire is going to lose at Eriadu because they're overconfident and they underestimated their enemies, but if they brought the full force of their military to bear on the Ssi-Ruuk... the Ruuk probably aren't going to have the ships and resources to take on the whole Empire at once. That might mean that in the wake of Eriadu, the Ruuk start trying to keep the Empire off balance by weakening Imperial forces along the border (so the Empire gets paranoid about the Alliance or Mandalorians or Zeltrons or whoever trying to annex more territory, etc), or poking at the Empire from different sides and different directions to try and keep their forces distracted and divided. That lends itself to trying to avoid antagonising the Alliance as well: if you're after honey, you go after one hive at a time, you don't want to stir up multiple swarms at once. If they're trying to conquer the galaxy though, out of a sense of superiority and divine right (or whatever), that would likely lead to a different approach. If they came to Terminus with the intention of crippling the Empire, they're going to want to do so swiftly and decisively. They might go after Fondor, Kuat, and other worlds like that to try and cripple the Empire's military industry, or target worlds like Carida where Imperial forces are trained. However, if they want to cleanse the whole galaxy, leaving the Alliance alone - giving them time to learn, prepare, build up their military, exploit the Empire's weakened state, all of that jazz - might not be the best strategy. They might want to provoke the weaker Alliance into being reckless and engaging them head-on (something that the Alliance is far less equipped to do), by targeting worlds like Ithor or Chandrila that the Rebellion "abandoned" to remain part of the Empire. They may try to use infiltration via Clawdites and what-not to trap the Alliance in political gridlock, turn factions against each other, attack vulnerable worlds just outside Alliance space so their do-gooder nature keeps them focused elsewhere, and things like that. It would be a very effective strategy, and plays into the idea of manipulating the weaknesses of inferior species so that they bring about their own demise... but it would require a LOT of knowledge and understanding of Imperial and Alliance politics, which the Ssi-Ruuk wouldn't necessarily have - unless they have a Locutus of Borg equivalent (I have a guy who was at Vanguards who might work for that) that is providing them with that sort of insight. It would also mean that the Ssi-Ruuk are a much, much bigger military threat: if they're able to take on the vast military of the Empire, and take pot shots at the Alliance as well, that might stretch credibility a bit. We probably don't want the Ssi-Ruuk to be a medium-sized threat, not impossibly vast: otherwise we're basically just writing the Vong by another name, and I don't think anyone wants that.
Just an Idea about where these Refugees can Go. Gerrenthum is the Capital of the Anot sector. Its a major spaceport for the area so makes sense that would be a good spot.