well, in SMRPG the star pieces were parts of the literal road to star haven which granted wishes, whereas the star spirits in PM lorded over it and granted the wishes. I would say if anything they might be related in location, but not purpose or character. Unless there is a higher being than Rosalina, I think the theory that Geno would work for her makes sense, but at a higher level than any luma (as he had the ability to form speech instead of just star noises and had more power than being a launch star) yet, if there is a higher being than Rosa, then I think Geno would work for them, and be equal to her.
Considering the PM development being intended to be SMRPG2 The theory that Geno would work for the star spirits makes sense, but I find that with the gravity in the SMRPG narrative, if PM truly would've been SMRPG2 then it would have taken a much different route and not have bowser be the main villain, more like Super PM than the OG PM.
As far as which villain Geno would be most antagonistic against? Going off of the justice vibe, probably Ridley or Ganondorf. They each seriously damaged their universes, of which Geno would be really against.