We have divided opinions here
"Shoals"
"Solos"
I am sitting right on top of that fence, so I'll bounce over shoaling first, and move on.
I can honestly tell you, piranhas LIKE being together!! I have 3 small babies in my 75, 2" 1.5" 1", with 4 feet of tankspace, and you know what? They're ALWAYS together!! One will wander off for a bit, and it'll be back with the group five minutes later. They are not "forced" to group, they do it because they like it, therefore, it is a SHOAL of juvenile piranhas.
Now that that's out of the way, I would like to address the question at hand. A group of piranha, not a whole lot of anything will make it permanently.
However, with a solo, the piranha is more likely to attempt shoaling with it (which would indicate that they enjoy company) than eat whatever hits the water, IF it is pellet fed, and fed well.
I have a solo RBP, if you have an issue with that, I don't care, he's physically incapable of shoaling with the others (size difference and half blind) He's been doing fabulously, he's not stressed, he's active, friendly (for an RBP) and healthy. He eats well, twice a day he's fed cichlid pellets, and I have successfully kept plecos with him, until the plecos were being mean to him, and right now he's living with my 2 inch pink con (the RBP is 5 inches)
That pink con was supposed to be a snack for him, since it's chased 2 fish up into my filters. And that little snot is perfectly fine. For once, it doesn't harrass him, and he doesn't bother it.
Now, before you get to thinking this is normal, I'll remind you that my RBP is half-blind, and can't be with other piranhas, so he'll take whatever buddy he can get.
I agree that piranhas should be together, in groups of at least 3 (no even number BS here) and should be close in size. They do best with "like" fish. Which means, fish like them. SD, Pacu, Exodons, they are behaviorally similar. So if you don't want 3 piranhas, you could probably sub them for the other 2.
Chances are, your fish will be happier that way, rather than completely by himself.
And why is that? Because I'm gonna beat the tar out of a dead horse and say this:
PIRANHAS -LIKE- BEING TOGETHER!!