There really is no such thing as the 'perfect structure' - it's just that fewer members make for easier management ultimately. it is easier to coordinate 10 players than 100, particularly given the global nature of this game.
Essentially the founding of a good tribe is someone who has taken a 'Management 101' class, and then a group of players willing to listen, learn and most importantly be led. You don't even need that much skill - you just need to want to learn, and that sort of attitude can be the difference between good tribes and bad. Leading a tribe is no different than managing people's careers - delegation, time management, growth, reward. A leader needs to be the chief communicator and bring the tribe together. You can of course have more than one leader, but this tends to make things inefficient. The only benefits to havin 2+ leaders is that because this game doesn't stop, more of the workload is shared.
And the thing that binds the above together? Activity. If the leader is inactive, the tribe has no direction. If the tribe is inactive, the leader can't drive the tribe anywhere. It's a team game at the end of the day, and the sooner people learn to work together, the more effective that tribe is. Activity, or lack thereof, above anything else is the killer of any tribe, in any world. You don't even need to know how to send a sub-sec train, snipe or some of the more 'advanced' skills as long as you are active (and willing to learn).
And I'm well aware that, that is easier said than done