As a big-time geek, I've read most of the Halo books, as well as memorised all the games. There is definatly enough material on the covenant side to create a race, and on the UNSC side as well- but you would probably have to make do with one or two ships per size- Check the following two pages:
You could reasonably get perhaps 5 races out of this- The Covenant (The Elites), The Covenant (The Prophets), The UNSC, The Pirates, and if you were feeling adventurous, the Sentinels (They were the pesky robots from Halo that alternated between killing the Flood and the player. The book Halo: Ghosts of Onyx showed that they are capeable of combining into larger and more powerfull robots, capeable of taking down a Covenant cruiser)
Yeah, I'm such a geek.