As long as your 55 is 48 inches long, then you can keep pretty much any fish you want in it, tangs included. Just be careful not to overstock, which is really easy to do in SW.
Before you put water in the tank, get a good book, like John Tullock's Natural Reef Aquariums or Bob Fenner's Conscientious Marine Aquarist. For less than the price of most fish, you will get a much better idea of how to start.
It may be a broken record, Dave, but it is good advice. Mike, look for those titles online, usually you can get them at greatly reduced prices through Amazon.com, Ebay, or other sites. Someone here found Fenner's book for around $25 I think.
yes...i just got a 15 gal. and i have 1 algae eater, 2 guppies(pair), 2 swordtails(pair), 1 puffer, 1 platie(mickey), 1 glass fish, 1 tetra(don't know what kind), and a mystery snail.