Yellow tangs school in the wild. If you have one that has established its territory, and you add one more - you'll still end up with one :-o. Add 3+ more at the same time and you're cool. Sometimes better off as tangs seem to be quite sociable. This is, however, not the case with angelfish as they do not school in the wild.

Same applies for butterfly fish - they school in the wild (not true for *all* butterflies). But to maintain your tank with several large specimens, you need quite the rockwork, swimspace and a conflaguration of filtering units.