If it is definately only going to be a fish only aquarium, any fish is good, as long as the adult size is not going to be very big for the 55 gallon.
Fish can be introduced at the same time, and like the other person said, you need a longer tank if you are going to keep tangs, however, I have kept tangs in a 55 gallon aquarium witout any problems.
I hope this helped.