This is very true I once kept a group of 12 RBP together and although they loved meat they used to go mad for boiled broccoli and other such greens.Petco likes to mislabel...
Piranhas do have vegetarian tendencies. Mine ate plastic plants.
As far as putting an Oscar in there as well I wouldn't recommend it, I had one RBP that was an arse and would push the others around a bit, never caused any damage was just bossy. Well I was having a beer one night and happen to look at my tank just in time to see nearly all the other RBP converge on that bully and destroy him. He was gone in seconds just a cloud in the water.
They would make short work of an Oscar.