I don't think so. There is no tank large enough for your Pacu to be able to guard against Piranha attacks. Plus, you need some rather large tanks to keep either one (Piranhas school and Pacus get gigantic). I would recommend against these for the beginning aquarist.
Iridescent sharks are awful aquarium fish and should be left in the wild.When they mature you are stuck with a jumpy fish that can get HUGE.I've heard of people who have had to foam the inside of their tanks from the constant bashing. Although I have heard that they are quite tasty.