My first female A. sandaracinos (orange skunk clown) got to about 4". I had her over 5 years before she jumped out of the tank. I put in a smaller skunk and the male became a breeding female in a few months. They have been together probably 5 years and the female is about 3 1/2" long. The Fautin/Allen book says 130mm which is about 5 inches, but that is bigger than I have ever seen in an aquarium.
For your second question, the answer in my experience is that you would probably be OK, but you should exercise extreme caution. I have a breeding pair of perculas and my breeding pair of orange skunks in opposite sides of a 48" 60 gal tank. In fact they are both currently guarding eggs and have been in the same tank with both breeding for at least 5 years. I have never seen any disputes even during feeding time.
Both my pairs are very into their anemones. Originally, I had natural host anemones for each pair that were not generally naturally accepted by the other pair. I had a gigantea for the percs and a H. crispa for the skunks.
I say use caution because clowns have personalities and you can get some really nasty females sometimes (of any species) that don't want anything to do with any other clowns and will chase them to the ends of the earth.
Order of introduction didn't seem to matter in my case. I think the percs were first by a few weeks, but I think it helped that both were introduced into the tank as pre-formed pairs.
