Plenty of fish and reptiles that are largely herbivores as adults eat more animal protein when they are very young and small. Perhaps pacu are in that category? I don't know, but the ones I have owned were always gluttons who would eat anything offered.
I agree that they may become less skittish as they grow, but in my experience a large pacu is one of the worst offenders when it comes to harrassing, killing, biting and generally chewing on other fish kept with it. They all turned into big fat bullies eventually.