One thing that I've noticed that should be considered regarding Trek and the Primary Weapons (Or Light Weapons if you prefer to call them that) is that, After TOS, (And the only reason this caveat is even in there was when Kirk used the term "Klingon Phasers" in Errand of Mercy) every race's primary weapon is also miniaturized into their ground weapons. Breen, Romulans, Klingons all use Disruptors as their ground weapons, and they use them as their primary weapon on their Starships. Cardassians use Phasers for their ground forces, therefore it makes sense that, even though it's never mentioned, that Cardassians would use Phasers as their primary Starship weapons. It makes logistical sense in that if you have a standard, and limited resources, that you would focus all of your RnD into one branch, rather than have two different research branches (one for your sidearms and one for your Starships), in which case you would specialize in neither, and find yourself behind other races in terms of how powerful your weapons are. Obviously you'd have some research in the other branch as you'd need to be able to counter other races that decided to go that route, but the bulk of your research would be in your specialized field.
So: I guess to sum it up. Cardassians use Phasers on their ships, because that is what their ground weapons are. As for their heavy weapons, I would agree that Photon Torpedoes seem the most likely, although we don't know for sure.
As an aside, the Romulans set up a field hospital on one of Bajor's moons towards the end of the Dominion War, they then proceeded to import a lot of Tri-Lithium to the field hospital, which (I think it was) Worf (maybe Odo) said was a key component of Romulan Plasma Torpedoes.