you know I have been really thinking about your commentt about what I am looking for well to be honest alternate races as well as the standard, not a full conversion mod but something that adds more to the game as I said races and more intresting planets maybe even ships that are well ballanced and work well with the races that are added to the game.
Well, Maelstrom also had the original races playable, in addition to the 3 new ones he created. However, in Diplomacy they were not that well balanced IMO, the new ones where far more powerful than the default. However, he said that he would address that problem in the Rebellion version, but I have not personally played that yet.
Star Wars: Requiem was the first mod that sounds like you are looking for. It added two Star Wars factions but the original races were still playable and the whole theme of the mod was that it stuck to Vanilla style gameplay. I used to work on that mod in Diplomacy, but I'm not sure exactly what the Rebellion status is other than its playable. Eventually I'll be making my own Star Wars mod that will also keep the original races but will be based off my Enhanced 4X mod rather than strictly Vanilla Rebellion gameplay.
I'd also highly recommend you check out ZombieRus' Fall of Kobol mod. It's part of his "Sins of the Fallen" mod package and has broken up all the races into modular parts. The mod adds two Battlestar Galactica races and you can pick and choose which of those 2 races or original races you want to play with. He also has 3 or 4 more races he'll be adding to that system eventually that he had in Diplomacy. 
