What up welcome to MFK. 3 Red's give them a few more months they'll get even bigger. I also keep piranhas any questions just post or ask someone who owns them for better insight.
The 55 is ok for a grow out tank, powerhead optional or set low they'll get some exercise. Too much makes aggressive and stress, caused by bumping each other always paranoid.
Light wise dim it out they do dialate can take amounts of light in. Lights left on on too long can blind them or create vision damage, since they don't blink but do have a thin coating over the eye. Bugging out eyes will point down then up & is a sign of light overdose.