Although a nice idea, which was actually part of the original design, it would actually be a pain to implement.
The premise of non-empire civs is that they can only have one system. That has mostly to do with diplomacy, and subjugation.
Notice however that there are three "types" of minor races - MinorRace, DevelopingPower and ExpandingPower. The latter 2 do have some form of "expansion", as their "territory" expands as that of an empire; an empire cannot colonize a system inside their territory without DoW, so the system is actually theirs, they just cannot colonize it.
Also, the higher the TechCurve of the civ, the harder it is to get treaties with them, including membership. The Breen and the Ferengi are TC5, right below the empires' TC6, so they're almost treated like empires (in this aspect, only the Cytherians are TC6).
Minors will be able to wage war against empires, BTW. There are still a couple of issues related to combat callbacks that need to be fixed before the AI can go on a rampage
(there is code already for the AI to attack systems, but it crashes the game...)