I always buy immature clowns, I add them all as juveniles and usually put them all in on the same day. If thet LFS has several clowns, I usally try to get 2 that are different in size.
By only buying juveniles and adding them all at the same time, I have never had a problem with fighting even when they reach adult sizes.
Note: What works for me may not work for everyone, take that into consideration.
I will try and add a picture later on of all the clowns together ....if they will cooperate.