I have a 4 inch female salvini with 2 severums (one 6 inches the other maybe 3), jack dempsey (6 inches), oscar (8 inches), and a green terror (3 inches) and the salvini is the lowest on the pecking order, but it claimed a territory in a tree root and is never ever unhealthy. It is even afraid of the littlest severum though, but it has also been housed with a texas and a nicaraguense before and it was fine too.
They are smart and they hide, but mine isn't really over aggressive, just territorial. Very different from the severums or the oscar, they kinda just swim around all the time.
btw mine is in a 75 gallon (yeah its overstocked, but I am getting rid of the jack dempsey as soon as I go to st louis)