Well, as far as I know, the EU states that the Mon Calimari were a race oppressed by the Empire, and that Admiral Ackbar served as a slave under Grand Moff Tarkin himself, which is where he learned a lot of his own military strategies. I can't recall exactly when or how the Rebellion assimilated the Mon Calamari into the fold, but it was considered a major coup for them, as the Mon Calamari cruisers bolstered their fleet considerably, and Ackbar was apparently the first Admiral the Rebellion ever had.