In the past, I've had the best luck mixing clowns when there is only one individual of a particular species. I've kept a tomato, sebae, and true perc. together successfully in a 55 gal. in the past with very little agression or fighting. If there was two or more of a certian species, well, that's where I would run into problems. Certain species like maroons, on the other hand, are much more aggressive and don't mix well with other clowns, paired or not. But just like mixing any species of fish, it's always somewhat of a crapshoot, and there is always a risk of aggression.
MikeS