55 should be fine...... I love the size my cons are right now, about 2 inches i think. Hard to tell in a bow front, makes everything look hugeeeeeeeee.
As for sexing the cons. Even as small as 1 inch you can tell. The female will have orange on the belly, and above the mouth towords the fin the male's head will have a slight hump and her head will be more sloped. I was at the LFS today and could sex all 12 in the tank and they were no bigger than 1 inch.
Good luck.
D