Starbase missions have different enemy groups depending on what tier the host player has gotten his starbase to, so enemy factions will increase in variety as we level up over time.
Colony invasion however does not seem to have any restrictions so we should face any enemy group at any time. In saying that I have not seen any Cardassians, Romulans, Orions, Gorn, Borg or Nausicaans at all so far.
Unless I have been incredibly unlucky could these also be added to help spice it up some more?
I'm not so sure about that, I ran into Undine in a Colony Invasion map last Wednesday and it was negative fun given how many were healing themselves back up to full HP while being shot up by multiple people. It was a PUG so I suppose someone could've been from a fleet with a T1 SB already but if that was the case, and if your info is correct, then it makes me wonder what kinds of stuff we'd get thrown at us with higher tier SB.