Darth Subjekt
The beginning of the end.
Well to hear lucas tell it, if you look back on history, the lower tech civilizations were the ones winning wars, ie the fall of the roman empire, Vietnam kicking our ass, even our Revolutionary War. So the concept of a primitive people defeating a technological giant appealed to him. And you forget that the alliance was started by Senators with lots of money, and rich homeworlds. Bail had money, and must have been some type of King if Leia was a "Princess", and Padme and her connections with the Naboo royal house...not as much resources as the Empire, but not bad for an underground rebellion.
And then by 6, they were prolly gaining more support and would get whatever people left behind or gave them. I'm also thinking maybe some of them took some kind of money from Tattoine after they killed Jabba.