As has been stated on this board a lot of times, there are no sure things. Its not a sure thing that he's going to be aggresive, its not a sure thing that he's not. A good friend of mine has one in a 55 gal. tank with a clown, a mandarin goby, and a couple of damsels. He swims around quite a bit, is BEAUTIFUL, and comes to the front of the tank when you walk up. I'm sure there are some instances of a six causing a problem, but you should be okay.