Aggression peaks when they get to sexual majority age, they fight over who gets to be the female. The idea of getting them young is that they sort that out when they are young and there isn't a huge fight when they are older and more dangerous to each other. Small clownfish can't really hurt with their bites, mature ones are often reported to make their owners bleed when doing tank maintenance.
Setting up for the sorting out is the long-term plan of the harem game, the rock work should be arranged such that lines of sight are broken up relatively quickly, if they don't see each other they think they are alone. This will give the weaker clowns a place to hide from the dominant ones.
Divide the number of clowns by 2, and that's the number of isolated areas you should try to establish in the tank. These can be anemones, or walled areas of the tank, basically hiding spots. Again, the more visually isolated they are the better.
