Generally, Hutts held ranks within the clans. Various clans held more or less power depending on their powerbase. Obviously, larger clans had more power and more assets while smaller clans lacked those assets. A non-military rank system could work, but honestly, sticking to the trainee/adept/elite ranking would be far more efficient and less confusing.
I would say register each Hutt under a clan and have one chairman head up a council. I wouldn't say they'd be able to unite the Hutts and get them to do whatever they wanted. We're Hutts. We do what we want, when we want, and bugger the consequences because we can blackmail, bribe, or bully our way out of them. I'd say have the Chairman have an ultimate Veto power to a council approved idea or plan, but can't submit their own policies like the rest of the council can. That keeps the a series of checks and balances on the lead Hutt and the council.