[ponokareefer]

Are you talking different kinds, or multiple clowns of the same species?

If it is the same species, add them all at the same time, and you'll be fine. If you are adding some in after the fact, add smaller ones, and try to re-aquascape some.

If it is of different species, it is more difficult. It can be done but you have to do a lot of things:
-try to add them at the same time. If not, then and try to re-aquascape some.
-have a long enough tank, minimum 4 feet. The larger the tank, the more likely you are to succeed. What size of tank are you attempting it with?
-get less aggressive species, such as percs, false percs, skunks. If you try Tomatoes or Cinnamon's, you are going to have a very difficult time.
-cross your fingers, and be prepared to get some out if the aggression will not stop.