Yeesh--
Emma is right (even though she says it in an indirect way, lol

) Arguing the whole time is not helping this person who originally posted make a decision any easier. But we can agree (for the most part) to use caution.
To whatsafish7: Perhaps you could just add one pirahna in at the beginning, just to be safe. But also, Even though these fishes may not grow to full length, they also very well MAY, and have been known to. SO just keep an eye out and be able to determine how much space your fish has to himself. Be Honest with yourself. When and IF you feel like the fish is getting to be too big for the current tank you have, it is the wisest decision to provide a larger more permanent home for the little big guy
I'm sure if you went with you instincts and extreme caution (as I am sure Pirahna's are quite the distinctive fish) I'm sure your Pirahna will do just fine
Good Luck!