On the subject of the White Comet Imperial Navy, versus the Garmillas Astro Navy, I'd put this fowards as a theory;
There is a definite gap between Garmillan naval power and whatever the White Comet Empire can offer--in fact, I'd go as far as to surmise that even with the Garmillan Civil War (unless it drags out for a really long time), Garmillas maintains that strategic advantage.
However, there are a few things we know about the Gatlanteans, at least from the production staff in post-2199/post-film interviews. The Gatlanteans aren't a colonial people (like the Zaltz are, and the Alterians were, grimly), though I'm pretty sure the two greenish-skinned women we do see in the series (one is Golick's slave girl, the other is a servant in the imperial palace) are Gatlanteans, as are the green-skinned males on the prison planet. The name's not just posturing, they do apparently have something that qualifies as an interstellar empire based out of the Small Magellanic Cloud (Gutaba province, though the exact geographic boundaries aren't given in the film). So they're not coming from Andromeda (you have to imagine that any power that could come from another galaxy and invade, like in the original show, would be nigh-unstoppable to anyone but our plucky heroes), but instead have a similar arrangement to Garmillas in the Large Magellanic Cloud.
It's not clear how long they've been fighting the Garmillas. Personally, I think it's a somewhat more recent development, if only because prior to the Yamato showing up, kicking ass, taking names, etc., the Garmillan Astro Navy would have wiped out the White Comet Empire if they had a hundred years to do it (turning the Gatlanteans into something like the Zaltz, if not exterminating them totally).
I don't think they're "tech thieves" in the strictest sense. Don't get me wrong, there is definitely a high level of reverse-engineering, and I doubt even Dagam would make up the whole "science slaves" practice for rhetoric only. But it's hard to picture that the White Comet Empire could have derived all their technology from Garmillas, or very recent finds from lost civilizations. They wouldn't have an actual empire, albeit a small one, if so. At least to me, it makes more sense that they already had a small empire prior to their war with Garmillas. I think the Gatlanteans, to some extent, a hard-nosed and pragmatic--they realized another interstellar empire existed, and in the handful of battles they could ilk out a victory in they tried to adopt as much as their technology as possible, which is how we get the force we see in-series. It might still be of inferior quality, they might have less nautical skill overall, and they certainly seem to be outnumbered (the Gutaba Expeditionary Fleet from the film seems to be the premiere military force, but is peanuts next to even your typical Garmillan fleet group). There might be some other factor that keeps the Garmillans from completely wiping the floor with them utterly (aside from bad timing--maybe the Gatlanteans are very good at repelling planetary invasions, and Garmillas is particularly interested actually occupying the White Comet Empire's worlds without totally obliterating them? Strategic constraints?).
"National polity and hegemony," as their head of government claimed (I'll repeat that ad nauseum I bet). The White Comet Empire has some imperial ambitions, obviously, but they're not the unstoppable force of God they were originally, or they probably wouldn't have lost two battles so catastrophically. With that, you actually get a force that could potentially pose a threat to the UN Cosmo Navy come next season, while not have upsetting the balance of power in the first. Of course, come season 2 the details could roll out and prove me entirely incorrect, but at least in my mind that kind of works out.
