The setup reminds me of ancient Japan. Granted there is no emperor in this setup, but there is a Shogun with several Daimyo warrior-lords vying for power beneath him. Everyone is nominally loyal to the heirarchy, but power is power and how much a Daimyo has can determine just how loyal he really has to be when it counts.
Any such names would work. Emperor and Kings, Shogun and Daimyo, King and Barons ... what we're talking about here is essentially a feudal system in space. Awesome.
One important thing to consider is where or not the leader (Hegemon, Emperor, King, President, whatever you choose to call it) actually has a territory-holding title beyond his administrative buildings. If the primary leader actually controls Sol or even the surrounding sector itself, the story would be very different than if the title only granted whatever administrative and military complexes were necessary to fulfill the role of keeping everyone together in an alliance (and is therefore constrained to relying on income/supplies in the form of taxation of his vassals.)