the general rule is 20 gallons per piranha so 6 of them would be 120...
i have a shoal of 6 in my 75 right now.. but they are just shy of 2 months old and will be moved to the 125 as i see fits..
if you going buy a tank to grow them out you might as well buy the biggest you can afford and just leave them in there for life, a 120-180 should be fine but so would a 400g lol
I would have to agree that 120 is best for life, but a 75 will work temporarily. just figure out what ur plans are, because they will quickly become too much for a 75.
you might even be better of just going for broke and then you will have to worry a lot less
you could probably find a few larger tanks than a 75 possibly with pumps, heaters, lights, used on craigslist. i'd try that first. 75's are pretty pricey for the size value.