Any time a Clown Trigger is in the mix; there will always the potential for war. CTs can play nice, sometimes for years, then go postal and kill or maim everything in the tank. These are gorgeous, fascinating fish; but, after several "experiments' over many years, I'm convinced they belong by themselves. (at least long-term).
Also, I'm not sure "stress free" is really possible, especially with larger fish. If you QT everything from day 1; you'll never have to worry about stress causing ich, or any other parasite, to appear.