Weaning piranhas onto pellets is quite a strenuous process if they have been raised on frozen or live for majority of their lives. It took my Sanchezi a couple months before he would even look at pellets as a food source...needless to say he was spoiled on tilapia, shrimp, and other frozen fish.
I don't know how difficult it is to wean juvies onto pellets, but I would assume it is better to start sooner than later. You just have to stick with it. Piranhas will not starve themselves to death, so one of two things will happen: either they will eventually go for the pellets and recognize it as a food source, or they will resort to cannibalism to satisfy hunger. Obviously pellet training with pygo juvies is a gamble, provided the expected thinning-of-the-shoal is a regular process, and then also pellet training might cause an increase in those chances of cannibalism.
The results, however, are very beneficial and can be a rewarding achievement once the process is complete. Good luck my sir.