Welcome to the board fellow reefer. Yea, you can over do it. IMO, it is really a question of how much room you have. Many LPS corals for example can "do battle" with each other if they are within say 3 inches of each other. Such battle comes in several forms, but the point is that they will compete to the death for space. So there is a limit, and it depends on the aggressiveness of the corals and their size.
sam