Hey Captain, I think ZeroAdunn is right on the Cardassians. I swear that I heard on a DS9 episode that their Defense Plateforms used Plasma Torpedos. But that was the only time I have ever heard the Cardassians using Plasma Torpedoes. No clue if they ever used them on ships or not. One of the Voyager episodes (I think it was called "Dreadnaught") had the Cardassian ship use Quantums, but you know my opinion on Voyager episodes.

I would prefer to stay away from using references from them as much as possible. Actually, I don't mind your idea Captain for Plasma's. I think it would be a good idea to just keep the Romulans using Plasma and have the tech easily developed by other races if captured. All ships use Plasma streams anyway to power their ship (even the Enterprise). I think if a Race captures it from the Romulans, they could easily modifiy it to their vessels.
Question though about Romulan Plasma's. Are you going to make them Direct Fire weapons, or Seeking? Also, I would advice making Plasma's distinctive compared to say Photon Torpedos. For example, what makes the Plasma over useful over the Photon and what would put the Plasma at a disadvantage to the Photon?