Hi Beach..
I started with 4 green chromis myself. They are active and not aggressive at all. They are kind of common though. Percula clowns are nice too and colorful, plus they are interesting. If you go with clowns consider some of the other species. In a 55 you can get away with just about any of them but it depends on what you would like to put in down the line too. I happen to love the
yellow striped maroon clowns. They are awesome in color and very personality rich. They are a bit more expensive, running around 30 bucks each and they can be hard to pair though. Later on, you will need to watch what you add since they can be very territorial, see the comment on Damsels
They are better than a velvet or blue damsel though and don't go with a domino.
You may also want to consider a nice goby or blennie. Some of these are really cute, they have interesting behaviors and are very colorful. One of the more interesting combinations would be a goby and
pistol shrimp combination. Your tank is a bit young for a shrimp at this point, but you can add one later even if you have a goby in there. They will probably take up with one another.
The royal grammas are very colorful too.
The choices are many and it really all depends on tastes and future plans. If you plan on corals or mushrooms later, you need to make sure you don't get anything that is going to eat them. Watch the max size listed for the fish, you don't want anything that will get over 6 inchs or so in a 55. Unfortunately, that would rule out a tang, but those are really fish that are left for a more established tank.
You also want to keep in mind a function that the fish will serve in the tank. A lawn mower blennie will help with algae control and some of the gobys will help with your substrate.
Wait until your tank is cycled before adding any fish, no reason to risk ammonia deaths at this point. Give it a month or so to settle out, track your water parameters and then when things are settled, go for your first fish.
