I wouldn't go with anything less then a 75 gallon if you want to keep them with other fish. These guys are very territorial, and their territories are the whole tank. A single specimen will go with other large and robust Tangs,
A single pair would need a tank to themselves, but you can get away with a smaller tank. The best way to get a good pair is to get a hold of a few of them and let one form - Just buying two random fish won't do it. The extra fish will need to be removed of course.
They're substrate spawners, so sand is best, along with a moderate amount of rockwork.