Ed, your observation on the difference between species is rather interesting. Pterophyllum scalare are of course commercially raised, although one can obtain wild caught fish from importers. P. altum is wild caught, unless one buys direct from a breeder. I would wonder if the aggressive tendancy of P. scalare is yet another example of what occurs with generations of inbreeding? Several long-established species of commercially-bred fish are showing signs of various issues from aggression to poor immune systems to considerably shorter life-spans, and biologists don't doubt this is likely due to the isolation of the gene pool. Food for thought. B.word of caution with angels in shoals.
this seems to work best on large tanks 75g + with 5 angles.. keeping a close eye on them as they mature.
angels are prone to pairing up and the aggression level rises to a level where angels in the tank can be in danger.. you may need to do some moving of the fish in smaller tanks as a pair will take over a large portion of the tank.(they are often better off moved from the tank)
but if you can establish a shoal of angels it is pretty impressive to watch them.
btw, the only angels I have had success with doing this has been P. Altum.
I seem to always find the odd overly aggressive P .Scalare.