This topic is constantly debated. When thinking of types of fish and what size aquarium they require, you must think beyond the size of the fish. Think of the fish in the wild and it's behavior. When I was 13 I went with my family to Hawaii and for the first time went snorkelling. I got into the water and the first fish I saw were achilles tangs in a school of hundreds!! Tangs in the wild usually travel in schools, and cruise the reef, stopping here and there to graze on algae. The point is that they really CRUISE. In contrast think of other types of fish, like the clownfish. Clownfish are nearly stationary, never straying far from their "base". Clowns act the same way in an aquarium, however tangs are less able to do that obviously. a three inch tang and a three inch clownfish will require a different size tank to THRIVE.
The next issue is the difference between THRIVING and SURVIVING. One example always used is that you can keep a great dane in a closet, but will he thrive? I believe this example to be a bit overkill, but it is a good example to show the difference between thriving and surviving.
IMO, a small tang of certain species can very successfully be kept in a 55 gallon, however, there are NO species that can live a FULL life. This means eventually they will require a larger tank, and more often than not this does not happen. Therefore I do not recommend it to the average or beginning aquarist. Keep in mind that most saltwater fish, if taken care of, can and will outlive a dog. This is rarely the case, but this is due to many reasons, none of which are limited by the lifespan of the fish.
With this said, there are HUNDREDS of options of species that will THRIVE in a 55 gallon. On the topic of Centropyge or dwarf angels, most of those species will thrive IMO in a 55 gallon. However, while they do feed from the water column, they are grazers also, picking all day around for things. Therefore it is best to wait a few months at least for the things to pick on to naturally grow in the tank.
Finally, we must always distinguish between what COULD be done and what SHOULD be done. Put on the Discovery Channel or watch a diving show and look fish in the wild. this will give you an idea of their behavior and what it will require in captivity. Some good next fish would be a pair of clowns, a dottyback or gramma, cardinalfish, gobies... just to name a few. Try and stay away for now on the "pickers" like the dwarf angels. Good luck though and you are on the right track.
