I though this might get quite a few responses so I have slit the topic to keep things on track
As far as your question is concerned, it really does depend both on the tank size and the species of clowns. I know a few people with large tanks (225+) that have mixed species and still have fighting to some degree. I think if you where to mix two less aggressive species you would have better luck but not less than a 150 gal
tank even then.
I would be leary of mixing clarkii's, maroons, tomato and cinnamon. They are very aggressive as a rule and should not be attempted with any other clown species.