At the end of the day , an Altum is a big Angelfish. They share the same behaviors. My experience with both Altums and P. scalares is the scalares are more aggressive. Probably because they are more likely to bred in captivity so you get the related pairing aggression in groups of them.
I did have a tank of Heckels with Altums and honestly did not like interactions in that tank.. young altums are food mongers like their scalare cousins.. Mature altums, seem more relaxed than the juveniles.
Behavior aside .. I really did not like the look of Altums and Heckels together in the tank. Its obviously subjective but the bars clash.... may just be because of my photography back ground but Looking at that tank bothered me.. whereas. Looking at a tank with either species alone was always soothing.
Al