fasturtle, your tank is about the same size as mine, and what you need to look at in buying a fish or invert is its adult size. One of the tangs---the vlamingi, I think---tops 18 inches when grown, and you can imagine *that* in a 55! Don't even ask how large a 'territory' they want to command!
My own rule of thumb is fish that top out at 2" to 3", so they have plenty of room to run about and establish territories and politic with each other: they each have their hole, their nook---except the silly chromis, that runs endlessly hither and yon looking for handouts---and the dartfish that sits midtank and defends his spot from all comers. The tailspot blenny is a hoot: he likes to sleep in a coral at night, but comes out to clean anything that's grungy, and isn't afraid of the dartfish[firefish] in the least. The two highfin gobies live in a hole, and only one at a time comes out; and the yellow watchman has a burrow he sits in front of like an old geezer on the front porch, all day long. A combo of swimmers and sitters is good: a few that eat algae are very good because they take away something the tank produces that you dont' want, and that means their food 'footprint' is very small on the tank. They're practically free, as far as fish load goes. The blenny is an example of that.
Urchins are fun, but you really have to secure your rock, sometimes with reef putty, because they can shove like you wouldn't believe. I think everyone should have an urchin once.
Conches burrow in the sand with just their eyestalks up, and an elephant trunk that feels around looking for food. They're hardy, and good sand-cleaners. Just 1-2 tiny ones per 55g tank, or they'll starve.
Peppermint shrimp are hardy and don't bother much: they may take an experimental nip at a coral, but rarely repeat. Coral banded shrimp are pretty, but a bit of a PITA due to attacking things.
Avoid crabs: cute, but fast growers and inclined to take to a diet of fish when they grow up.
My advice would be to meander through liveaquaria's site and see what there is, then come back to RC's "reef fishes" forum and the "invertebrate" forum and ask people's experiences with it. You've got a lot of time to make good choices. My own 'first fish' [I've had many tanks, interspersed with moves across country] always tends to be a yellow watchman goby, because they're so funny, but there are a lot of good 'first fish' in the sea! There are also real neat inverts like coco worms and feather dusters that you can have if you don't have a worm-eating fish. Etc.
One nice thing about the corals, btw, is that you can trade bits of them for pieces of someone else's type, and they grow FAST, contrary to rumor. This last summer, after 6 months' growth, I was fragging off pieces to trade at the rate of 1-2 a week. Fragging softies just involves a razor blade and a bit of fishing line to tie the piece to a new rock, which it will grip in a few days. Between Cheyenne and Denver, you can likely find a lot of people to trade with.
