well apparently I am no expert but many ppl say the rule is 20g per fish, so in a 50 g you could get away with 3. Its just that disappointing thing that when you first acquire pirhannas they are usuaally quite small, and 3 pirhannas in a 50 (or in my case a 60g) tank makes them look EXTREMELY TINY, but they grow quite rapidly with regular feedings. I'm using a penguin 350 bio-wheel filter in my 60g tank and a Aquaclear filter in my 25g tank, (which I plan on moving to my 60g tank once I finish the transfer for my pirahannas. For Filters, its whatever you are comfortable with, bio-wheels make a lot more noise, because of the water run off but I like them. Just make sure you get a filter that is a bit stronger because pirhannas give off a lot of waste, becasue of excess food that they won't eat etc... But I've been reading if you have live plants in your tank it should help with the 'nitrate' (??) levels, and you will only have to do water changes every week instead of vacs all the time as well.
HOpe that helped, and didn't just confuse you lol