If it's high quality models you want: I've collected a lot of those for SFC3 (ships and stations), converted quit a few from Bridge Commander mods myself. I've also polished them up (fixed textures that were a bit off, added break models, UI images, proper hardpoints, added illumination textures), because most mods just seem very unpolished and have fugly models/textures I wanted to make mine stand out in that department. Some of the models originally had 2048x2048 textures that I scaled down to 1024x1024 to shorten mission loading times and because you don't see the difference in SFC3, but I can help you find the originals on Bridge Commander Filefront. I have quite a lot of high quality space backgrounds as well. The space backgrounds are all mine so no futher permissions are needed, for the models you might have to contact some of the original modelers, or at least credit them.
You can use my work as long as you credit me in your ReadMe somewhere, it's pretty much all collected in one download, my SFC3 Upgrade Mod:
http://www.moddb.com/games/star-trek-starfleet-command-3/downloads/sfc3-upgrade-mod-30If you need an excuse to put more minor powers ships in your game, you could use the system I use in my mod: every empire (except the Borg and Ferengi) have allied minor powers (for example the Tzenkethi and Tholians for the Romulans and the Breen for the Dominion) and access to a few of their ships. This also helps you "fill up" those empires standard fleets if you don't want to invent (or worse, kitbash) a lot of ship classes that were never seen or mentioned in canon Trek lore (in my mod the Dominion and Cardassians don't even have 7 stock ships of their own, the Dominion heavy cruiser is the Breen Warship, the Cardassian dreadnought is the big Son'a ship from Insurrection).