Have her be Sailor Moon. We already have two, what's a third? She's just Serenity's predecessor.
And Shanami's
There's nothing in canon (to my knowledge) that says that multiple people can't be bound to the same source. After all, the Starlights and their Princess all come from the same planet, and all have a Star Seed.
I'm wondering if binding them to planets is really how it works even in canon. If I remember correctly in the manga, the Star Seeds come from the Galaxy Cauldron and just kind of end up on suitable worlds? Or something like that perhaps? I suppose I could look it up, but it's probably not all to important, because I figure I'll just go with this concept any way and make the worlds more like the place they were originally tasked to defend.
Though some how making her yet another Sailor Moon seems a bit... inelegant.
It's a possibility though, well how I'd prefer playing it, it isn't like the senshi form will show up all to quickly any how, so I guess I can think about it for awhile still.
Hm... just tossing out ideas here, but... Are you having Galaxia as another part of the same empire (considering all the senshi floating about in Stars...), or something else? If she was part of the same group as the Senshi, well... Galaxia herself has no objectives, it's Chaos. So she could've just been a soldier for Serenity's empire who got hijacked by Chaos - which, as you've established, works for the kingdom's enemies. She could be a remote under their control. Or Chaos could be going kinda rogue, which may allow an interesting bit with the antagonists working on shutting it down, and then going right back to shooting at the senshi.
Well, what I remember about Galaxia is, is that she showed up during the war torn period of the Sailor wars, or is that wrong? Which as far as I remember from canon was due to Chaos and there I've linked Chaos to another party, this would make it post Silver Millenium I guess. So she wouldn't be a Senshi of the old empire, but from one of the successor states instead.
So I guess the remaining question would be if for instance Chaos went Rogue, it certainly would put an interesting spin on things as well as explain its rather chaotic nature and why it isn't pushing for a more rational agenda, instead of its more destructive methods.
You could have the enemies be rebels who opposed unification (which you'd have to be careful with, since that makes the Moon Kingdom the aggressors, and makes your antagonists seem more 'good' than the protagonists - fine if you want it, otherwise best to avoid the implication).
But in general, you need to work out exactly what their objectives are. And why it's important enough to wage war over for over ten thousand years after their enemies are dead - if they just have different ideas on how things should be run, there needs to be a reason that it struck them as important enough to wage interstellar war over, and why they haven't started doing it in the ten thousand years they've had.
It's pretty much unavoidable that in unification you'll partially end up being an aggressor, so you'd need an explanation for why a peace loving state like the Moon Kingdom ever wound up like that. I think that won't present an all to difficult problem though. Something like ,constant warfare and other states attacking you could end up, with you pacifying and incorporating them in to your own state, just to get them to atleast stop attacking you. Or a reason much like that should work fairly well I suspect.
As for why they haven't done anything in ten thousand years and more, well I figure they have. But the situation is some what volatile with many different parties, so it would be much like Earths history, with many states trying to gain dominance, but always in the end falling a bit short, suffering reversals etc etc. It would certainly help explain the attitude of for instance Galaxia as well, in that she tried to take out Chaos in canon to bring about a more lasting peace. Hmmm, what if she decided to use Chaos to become stronger so she could force peace, except thing didn't entirely work out like she hoped...?