the more you get into CA/SA cichlids the more you'll find that in general males of the same species cannot be kept together unless in the largest of tanks. granted, that's a gross generalization and there are exceptions, but as a starting guideline it's pretty good to follow.
aggression in New world cichlids is usually worst when you're keeping a breeding pair...then no other fish are safe no matter how mellow the species is considered to be. one step down the aggression ladder would be when you're keeping 2 males of the same species in a tank that doesn't allow them enough room to keep far away from each other. the next step down would be when keeping 2 males of different species that are very close in appearance.
considering the reputation that umbees have and their potential size, even in a 450 or larger gallon tank I don't think you'd be able to keep more than 1 male. they are open water swimmers/predators and require a huge territory.
ps. please tell me you looked in awe for at least 3-5 min. at the picture of cichlidscene's umbee with his arm in front of the tank. when I first saw that pic I was in a state of

and couldn't take my eyes off the screen for several minutes.