If everyone starts out on the same team, does that mean that everyone's relationship is 10.0 at the start?
In 1.34 (Diplomacy), yes. And the other significant difference, is you have full vision, so you have a good idea of the layout from the beginning, rather than beginning in a dark hole.
Unfortunately, in Rebellion (on Steam), you can't start unlocked teams with all starting on the same team, and there no longer is a minumum number for allied team victory.