I have 2 Dempseys (6" inches and 7 inches) peacefully co-existing with 3 Aceis and 3 Yellow Labs half their size in a 55-gallon tank. All of the fish were introduced to the tank at the same time when they were 1.5 to 2-inch juveniles some 6 months ago and I have never witnessed the Dempseys bothering any of the other fish. There are plenty of rocks in the tank with hiding spots, but all of the fish swim about in the open freely. The Dempseys are not at all territorial and I have only rearranged the rocks once.
I know it's not recommended to mix fish from different lakes, let alone different continents, but this arrangement has been working great (so far).
The Dempseys will eventually be moved to their own tank.