I always took the Rebel Alliance at this point as being a loosely-organized, far spread group of insurgents.
Since the group involved in this skirmish in this sector of the galaxy is comprised (most likely) of turncoats from Alderaan, Dantooine, Tattooine and other such "human" worlds, I don't really see why it's necessary to throw in Rodians or what have you.
By the time we get to ROTJ (to me anyway), it seems like the Alliance has pulled together for one mass-attack. That's why you see more diversity there - everyone has gotten together to work as one mass space navy, rather than small guerilla warrior pockets (of likely less diverse groups).
Admiral Ackbar does not need to be on Yavin.
"Bah! It's a trap!"