Just thought I'd add something else. This may sound foolish to some, but I've had it work by personal experience. My silver arowana has been raised in a 75 gallon with a small angelfish. Well, he's about 14" now (don't worry he moved out of that tank recently) and he never touched the angelfish. He could have easily swallowed him. I definitely believe this is because he was raised with the angel from the time he was 6" and didn't consider him food anymore. Honestly, I think your combination will work as long as the aro doesn't far outgrow the pbass.
it is luck of the draw with aros and smaller fish. i have had aros eat angels that they were raised with and i have added angels to other aros without issues. all depends on the temperment of the fish.
