I likee to keep fish in pair or breeding group. In a 55, you need small fish that are mild in manner. Sleeperhead gobies are entertaining but they do stir up the sand too much. I use a DSB so often try to avoid them. Orchid dottyback are beautiful, bright purple. You can get them to breed in your aquarium.
Other fish that will do well are the smaller shrimp gobies with their shrimp. Yellowhead Jawfish. These are beautiful, will not outgrow your tank, and will do well in your aquarium. Another fish you should consider are Royal Gramma. I would keep 3 in a 55 gal. Make sure you get only one (or no male) The males will fight to the death, even in a 450 g tank like mine. The most dominant fish of the group will become male very quickly (days). Often when LFS keep single male in their store for a few days or a week. These should be avoid because they almost always turned to male and will fight when you get them together.
Flame, Fireball, Fameback or Argi (Cherub) angels are always nice but small angel in a tank, there are always possibility of coral pickers. Great information on Pygmy angels can be found here:
http://reefkeeping.com/issues/2007-09/jf/
Why do I like them (all the fish above)? They are small, beautiful and will do well in a small reeftank. Please don't put all of them in but just 3-4 more fish to your tank only.