Body shape is completely wrong for a baby piranha. It's too long and the fins are too large for a baby piranha. Even from the poor pictures, the mouth is also pretty obvious
Feeding it meat wouldn't help. Pacu will eat anything. Whether it's a piranha or pacu, it would probably at least try to eat it. It is however DEFINITELY a pacu.
Looks like a pacu to me. The corner of a pacu mouth is rectangular shaped, while a piranha corner of the mouth is an acute angle. Piranhas have a prognathic mandible--their jaws jut out. Pacus do not have a prominent lower jaw.