I believe with clownfish the biggest fish in the tank if there was 100 the biggest would be the female (or male i forget) and the second biggest fish would be the male. All clowns start out as males and then they change their gender to female if they are the biggest. So usually clowns are kept in pairs.
HTH
Jon-Paul
If you get two perc juvies at the same time over the next few months they will become a pair, if they are both males then one will become a female, she will become larger.
With the percs im not sure if the famales will become male but with some of the more aggresive species two females will continually chase each other out of there territory.
Or you could get a like 3" Clown and a like 2" Clown and they would probably pair up. I bought a 1.5" and a 2" from the same tank with about 25 otehr tank raised clowns and they paired up right away. They also hosted my sebae right away.