You have posted on a very old thread.
To answer your question, you can use a 55 gal tank for a SW system. However, IMHO it is not a good choice. A 55 gal tank is very narrow, only 12 inches wide. This makes it very difficult to aquascape. People have done it, but you really need to plan the layout so it doesn't look like a rock pile.
The tank is just about the least expensive part of a reef system, so I recommend that you consider getting another tank. My personal preference would be a 40 gal breeder, a 50 gal standard, a 70 gal, or 90 gal tank. Of course, you can always go larger. When selecting a tank the front to back width is what allows good aquascapeing. 18 inches is good, 24 inches is better. This is partly why you see a trend toward cube tanks, to get that front to back depth.