Clownfish, like damsels, are territorial. You'll need to provide a second "home" within the tank for these new fish to claim.
Ideally, in smaller tanks, you should only keep one of a species if you are going to keep multiple species together. In larger aquariums, you just need to redecorate a little so that there are places out of eyesight for the new fish to claim. The more places you can make, the more varieties you can keep with some exceptions. (Maroons, Tomatoes, etc.) Keep in mind that when you get small clowns of the same specie to start with, you are almost assured you will have a pair as they mature so you need to be mindful of this when keeping multiple varieties together.
Hope this helps...