I would also second 3-4 Chromis. I have 3 in my 60g and they are really nice fish.
Another suggestion and one fish that I am going to get later for my 60 is a coral beauty dwarf angle. A really nice 4"-5" fish that move fluidly through live rock formations. Now they are not 100% reef safe, but I had one in my 37g and it never picked on any of my corals. Most people would say that they are usually reef safe. they are really nice fish and a nice size for a 55g.
60gal reef in progress
75lbs Live Rock
40lbs Live Rock
AquaC Remora Protein Skimmer
Marineland penguin HOB Filter
1 - Percula, 1 - Blue-Yellowtail Damsel, 3 - Green reef Chromis
Rose Annenome; Anchor Coral