well, I have 12 red bellies, they are no bigger than yours, I moved them into an outdoor pond recently; they were really skitties at the beginning, or should I say they were afraid of human, and that was when I put them in the tank before; now groups of them could finish a frog with double in size in just minutes;
your RBPs already show some progress by " chomping the feeders in half ", just like you said, here, I think just leave them as it is right now, they will eat anyway; I had experience once with smaller RBPs, 7 of them, around 2" each, (just before they die because of the high chlorine level in the water, tht was sucks) just moments after I bought them, I put them in 50 gallons tank, considering the size of the RBPs, I put the tank on my terrace, with direct expose of natural light, the tank had no hiding space at all; the RBPs wont eat anything for 3 days, even when I put feeders in the tank, but on the next days I checked, my RBPs have finished all the feeders. Following after that, my RBPs would eat anything meaty I put in the tank with no problems at all.
just a little trick I did to get the RBP to eat at the begining, when putting feeders into the tank, I only put 1 feeder, cause that way would drive the appetite of your RBPs better,
as for the "size" for the same type of piranhas, I dont thing there would be any problem with that. Mine one was the same as yours; among my 7 RBPs, 3 of them were over an inch compare with the rest, but they got along nicely,
cheers