There's one more trick you could try. I've never seen it mentioned on any website or anything, but a friend of mine who has kept Cichlids for years taught me this. You could try putting the aggressive fish on "time-out," by putting him in a five gallon bucket for about a day (you should put a little airstone or something in there to oxygenate the water well).
Obviously the other methods (re-arranging rocks, decorations, etc...) are the first thing you should try, but the time-out seems to work by allowing those other fish to re-establish territories in there, and it makes that aggressive fish more like a "newcomer." I tried this out with a Venustus (which are already known as having a temper) and now he gets along just fine with the others.
One last note, that "time out" is rather stressful for the fish (anytime you're moving them around it causes stress), so really only use that if you've tried everything else and it's failed. Oh, and remember that you'll probably have to acclimate him to the temperature in the tank again! Also, don't leave them in that bucket for too long (a day is already pushing it), as ammonia could start to build up.
