Romulan Star Empire implies, to me, a stellar union. So does Cardassian Union. Maybe Morn's people are part of the Union. For that matter, the Klingon Empire probably spans several star systems and encompasses various species (we just don't see). I don't recaell evidence the Romilans or Cardassians were Xenophobic, just Orwellian and oppressive.
The Dominion obviously held dominion over multiple stars and species.
One would think the First Federation covered multiple peoples, but we saw how overblown and tending to exaggerate Balok turned out to be.
The Borg were multi-species and vast.
The Son'a had two subjugated species.