I tried to keep a pair of true perc's with a pair of cinnamon clowns. The true perc's were in the system first, and were larger. Everything was fine for about 6 months until the cinnamon clowns grew larger than the true perc's. The cinnamon clowns would leave their side of the tank and beat up the true perc's for a while, pushing them out of their anemone, before going back to their side of the tank. I tried the mirror trick with no luck.
I tried this in a 125 gallon system with about 150 pounds of live rock(lots of hiding places), and 3 anemones. Each pair of clowns had their own anemone, but the cinnamon's eventually claimed all of them, and I had to get the true perc's out after unsuccessfully trying to get the cinnamon clowns out. I wouldn't recommend trying to mix clowns in a system.