it seems that Lions are pretty laid back (at least my dwarf, antennata and radiata are very chill), also clowns, and damsels seem to be good tankmates. I could see a scorpion doing well too. I had read of a guy who had a large volitan with a broken wing who lived with a large peacock. The lion would always lay on the sand and the peacock would never mess with him....
I am always amazed at the new antics of my peacock and clown. Just a couple days ago, I was having trouble getting my peacock to eat a live crayfish (very unusual), It was in there for a couple days. About a day or two into it, I was home from class, and decided to stir things up a bit, and still the Boris was not interested-THEN the funniest thing happened... So, as the crayfish tries to retreat back around to a large tower of LR, the clown comes up and wedges himself between the glass rock and crayfish blocking the crayfish from retreating.... and sustaining it....at first i thought he was just swimming around, then after I while I realized he was INTENTIONALLY keeping the crayfish from escaping! It seems that the Clown knows that the crayfish are food for Boris, as I will see the clown round one up and nip at the a newly introduced crayfish! They are like a TEAM Now! It is really cool, considering when I first introduced the Clown, it was scared for it life- very stressed out, pale, and sickly looking for several weeks.... After changing its diet to a more herbacious one, and providing an anemone, and months of not being attacked, the Clown is now thriving! They make a very interesting pair....