Welcome to
Talking Reef !
Dick has already mentioned the dwarf lion and the damsel fish. I think an "allen's" damsel can be less tank mate aggressive and there are a few others, you would need to do some research in books/web to be sure.
I would be a little concerned about the Bursa Trigger in a 55g tank. Considering he could reach a captive length of 6" inches (10" in the wild) the tank may be too small for him as an adult. Triggers do like lots of swimming room and can be tank aggresive to smaller tank mates - though every fish is different with personality. As a juvenille he should be ok though give thought to his life as an adult.
Marine Depot is a good online source of information on livestock if nothing else. e.g. for your trigger
Bursa Trigger - Rhinecanthus verrucosus. WetWebMedia is also another excellent resource.
SufflamenFAQs.
Hope this helps!
Warm Regards,
