I watch a guy on YouTube, Dustinsfishtanks, he had piranhas. He said he thought they would be cool, but got really boring. He said they were super afraid of people don't know if you can get them to become less shy.
For live food, definitely raise what ever you feed on your own. Fish stores mistreated feeders even more than other fish, so feeding a fish like that will eventually lead to disease. What would you feed them anyways? I agree though, no putting it on YouTube,"piranhas eat mouse" such messed up. I know they gotta eat...
Build or buy a sump, building would be way cheaper and there are lots of videos on YouTube showing how.
15 Gallon NPT
1g no tech bowl in the making