can't agree more with the Borneman recommendation. An excellent resource. Corals are very aggressive - either physically as tk1360 points out, or through chemical warfare. Think about how, relatively speaking, there is limited viable space for corals on a reef, so they've developed various methods for eliminating the competition. You can mix varieties in one tank, but give them plenty of room, and watch carefully as they grow. Keep a log of weekly observations on growth and reactions. Make adjustments as needed. And as the others have wisely recommended - research THOROUGHLY before doing ANYTHING.