Hey welcome..im also from MA and i;ve been keeping piranha and many species of piranha for many years as well.
You have 55g now right? 3 is a perfect number for piranha to group for RBP. At that size you must feed them 3-4 time a day with small portion of food. Keep the food variety different diet each week you feed them. train them young is good so once they grow up they are not picky at food.
Also piranha dont like bring light so dimmed your light. plenty of woods and hiding space. Keep heavy filteration but leave dead spot for them to rest. Remember the more you feed them the more waist they produce. the more filteration the better.
55g will be good for them in a few months only. pygo tend to grow fast. once they full adult they will slowly grow. Keep in mind that piranha are unpredictable, doesnt matter how well you feed them or how well you maintain they will always fight and end up killing each other some day. the strongest survice.
keep the temp 79-82f for them, but also keep in mind that the higher temp the more active they are. The more active the more they will eat and most important is they will get aggressive with high temp.
diet keep in different every weeks.
young feed them worm, chomp up shrimp, and other small peice.
you can also feed the following food for them.
silver side
clams
market shrimp
scallop
fish fillet
pellet
smelt
worm
hope this help you a little.