Eh, given both the Cardassians and Romulans were highly xenophobic, I'd say a union is out of the line. Precisely for the same reason they're so similar at times, they're treacherous and expect treachery as the norm. So trust is out of the question. At best you're going to get collaborations like Tain and the Tal Shiar's incursion.
But if you're looking for a planetary union of multiple races... well the Dominion was pretty much designed on the thesis of being the "anti-Federation." Though instead of uniting together in peace and cooperation, it's politics out the barrel of a gun.