I wouldn't be so sure that the clown is female. If it doesnt have a reason to change, it wont. They are born hermaphrodites meaning they have the organs for both but if the clown hasnt been paired up, then it doesnt have a reason to change to female. Once it is female, it cant go back to male. The percs are not known to be as aggressive and territorial but you could pick one up who shows this behavior.
One question, How big is the existing fish? I would go ahead and just get one more that is bigger of course (so it doesn't get picked on) and then see where it goes from there. They will eventually pair up although it could take a while. A mated pair doesn't mean that they will spawn as soon as you get them in there. Just the new tank and surroundings could put this off for a while also. Another thing, just b/c they are mated doesnt mean they've even spawned before. A lot of stores sell them as mated pairs b/c they have been together for awhile and they show courting behavior. Don't let them take advantage of ya!!!
If you have anymore questions, let me know. HTH