Yep young RB pacus shoal with RB piranhas. Intriguing behaviour.
My experience is that pacus do not live that well with oscars though.
Apart from the feeding, pacus are very unyielding, and together with
territorial oscars they'll show how firm their bite is.
I had to separate them.
Piranhas are allready ferocious towards each other, and are therefor not to be
mixed with other species in a tank.
A group of oscars together is, because of their different characters, very nice to see.