Contarary to popular belief, piranha are not mean killing machiens, they are wimps
Place a hand or move suddenly near the tank and they run for cover, unless it is the keeper or family member...
A robust plec is armored, and will react to being picked on by shoving a sharp fin in the face of the agressor. Because it will be the dominant piranha that first has a go, the other piranha will see the dominat one backing off after being attacked and decide that the new fish is best left alone.
If there is going to be an issue, it will be at first introduction
Reduce the risk by feeding them up before introducing the plec, and leaving the lights out for an hour before starting it's aclimation, having blackened out the room. This will lead the piranha to leave the plec untill it is settled enough to stand-up for itself. It usualy works very well long-term, but there is always a risk. Watch carefully and be ready to get the plec out in a hurry if needs be
You shouldn't need to though
Plecs have a armored body, so there isn't much the piranha can chew, so they almost always never bother... Isn't worth their effort to kill something with sharp fins that keep being shoved in their face when they get near (that a large plec will do, they are semi-terirorial and will defend their position in a tank) so they just don't go there.
All the best
Rabbut