I definitly love the idea of having several (douzens or more) minor races. I enhance your idea:
Minor races are divided into several "classes". While the lowest class will only develop its homeworld and construct bases to defend it, the higher classes will colonize their homesystem or even advance one or two systems in each direction. That would be very cool imho!
I really like the idea of training admirals, captains, yada yada yada
Could be an offshoot of military science... Like make one planet your service academy... I'd love the ability to train command structures for my navies..