One of the deleted scenes in Episode III has a Mon Calamari senator, but it's unclear if it's Ackbar or someone else.

Those are female Mon Calamari dancers in the opera house scene, although you can barely see them. ;)

I always figured the Rebels had a lot of allies helping them under the table while keeping up appearances with the Empire.

I did often wonder why we had different leaders in each film. General Dodonna is nowhere to be seen in Empire, and we never see General Rieekan again either. And Jedi gives us three new leaders we never met before.

It was probably a bad idea if the entire Rebel Fleet was ever all together in one place unless it was for something big.