Pairs are best formed, fish sold as pairs from the wild can be old and unproductive. The best bet is to go to a store and observe a tank of a species and pick out the two that best interact. That is, neither is subject to attacks or attacking, they will be sexually neutral for the most part or already be setup in a heirarchy and know their place. This works for most excpet Maroons, the females are vicious. You may know but clowns start neutral go to male then female growing along the way. Two mature fish could both be female worst combo, look for small immature fish.