I think a Cardassian/Romulan alliance might be possible, but could never be long term. Both races are fiercely independent and expect betrayal and double crossing. Trust between them would be impossible so any alliance would be a delicate balance that eventually would get upset. Also, I think Garak probably sent the ships for more than one reason. The PR, spying, and maybe even manipulation of events are all possible. He might even end up helping the federation against certain Romulan factions if it is in Cardassia's best interest.
I think the Romulans are to fractured to be a fraction (that sounded like a rhyme... ) anyways I don't think they would release them as such. As far as I know the Cardassians are still recovering from the Dominion war and I dont' think they have the manpower nor the inclination to get into inter-galactic politics if anything I would think the Cardassians would try to join the Federation since Garak is known to support the idea of democracy and its a win win situation if they join because they can always leave the federation after they are back on their own two feet