If you are going to have more than one Tang, they should be introduced together or have a large enough tank, i.e. 150g or larger. Purple Tangs usually don't get along with any other Tangs and can be pretty aggressive towards others too. Tangs are open water swimmers who like large amounts of open water to swim. Tangs are more susceptible to Ick and usually require above average water quality. Tangs are also susceptible to lateral line disease.
Most tangs can get rather large. Most experts feel that Tangs should ideally be in at least a 100g tank and at least 6 feet long. Although people have had success with Yellow Eye Kole's in a 4 foot 90g tank. Naso's, Unicorns, Yellows, Sailfin's, Clown's, Hippo's all can get rather large.