My first suggestion would be to think about another size tank. Although 55's are readily available and fairly inexpensive, for just a little more, and taking up the same amount of wall space, you can get a 75 or 90 which will allow much more leeway in decoration and choices of fish.
If you look on craigslist, you can easily find a setup for much less than a 55 purchased new.
Do yourself a huge favor, and plan on having a sump with whatever size you choose. It is the perfect place to put all the junk you don't want to look at in the Display Tank (DT) such as heaters, a protein skimmer, etc. It will also allow for more volume. More volume directly relates to the overall stability of the tank.
Start looking at sites like liveaquaria for the types of fish you want, and pay attention to the needs of those fish. As mentioned above, adult size must be considered. That striking juvenile panther grouper (white with black polka dots) that is 2" today will easily hit 12" within a year and will eat anything that fits in it's mouth. Others, for instance many of the clownfish are very colorful but can (will) become very territorial and make adding other fish later very difficult. Many fish can coexist with shrimp and crabs quite easily, and in fact some (think shrimp/goby paring) will live together in a small cave in your rockwork. Others will happily eat all your shrimp the first day they are in the tank.
I'm not trying to discourage you in any way. Plan now for what you want your system to be and you will save yourself a lot of trouble, money, and lost creatures.