First off the pleco will get eaten, anything u put in the tank will die, and the fish even kill eachother so expect to lose a few along the way, 80gallons is adequate for 4-5 piranhas, but yours are still somewhat small, in a couple months you absolutely need to upgrade to a 150-180 gallons, I have 5 in a 90 and if I don't change the water every 5 days the nitrates spike, so you NEED 2x per week 50% water changes, piranhas are some of the most messy fish you can get, is the tank cycled?
General show is 15-20 gallons per fish
Anyways, get them on a food pellet (NLS,omega one) that will help them a lot and dont over feed, at that size one time per day is fine. Try talapia and shrimp too. No feeder fish!
Go S. Vettel #1 rb8