i'm not sure where feeders come into the discussion about adding additional piranhas period. I think what he really meant is make sure they're feed regardless of what it is you feed them (pellets, prepared, live) doesn't matter.
13 in a 135 is too many. he'll lose some probably over territory battles as they grow and take up more space in the tank regardless of how well feed they are. many people can vouch for losing piranhas on days they were feed.
I'd go with 8 for your 125 since you're already starting at 4-5 inches. Some people would consider that slight overstocking but you should be fine if it's overfiltered and you keep up with water changes.
The easiest way to add new piranhas to an existing shoal and many people can vouch for this is to make sure to feed the ones that are already in the tank. Reaquascape the tank to break up the current established territories. Add the new piranhas and then chase all of them (new and old) around with a net to scare the crap out of them. When you're done chasing them, they should be good ol buddies that have forgotten all about intruders being added to the tank.