You probably didn't see my thread from a few weeks ago. I had a 3" GSM and found a smaller one at a LFS. There was only about a 1/2" to 3/4" size difference. I put the new one in a clear container in the tank. The bigger clown went nuts and tried to attack the new fish for a few hours and then seemed to give up. By the next day they were swimming together, although separated by the plastic container. The following day I let the new one in the tank and the old one attacked it for about 5 minutes until the smaller one started fighting back and made the old one his/her b*tch in a few minutes. I tried my best but couldn't catch either one. It's been a few weeks now and the new one completely owns the anemone but lets the old one get closer every day. There's hardly any aggression and when it is, it's generally a nudge or nip on the fin, nothing too damaging.
I have no idea if they are male-male, female-male, or female-female but for now things are peaceful.