After discussions in the New World Cichlid OTP decided to go with 2 pairs of Rams. As a loose rule each pair needs 2 square feet with a broken line of sight between territories. If they ever breed they will protect their 2 square feet which means it's likely that all 4 square feet of the 55 Gal would be defended.
I'm sticking with my vote of 75 Gal
I also completely know what InchWorm is talking about. Every Aquarium I've ever had has been 'not quite big enough'. This explains my sig