Imho, 90g isn't an adapted volume for Pygocentrus nattereri, and 3 or 4 fishes don't really make a shoal.
You should rather choose a little/medium Serrasalmus species like sanchezi or geryi, or a medium sized rhombeus for some years, or a Prystobrycon like eigenmanni.
I would recommend 3 or 4 in a 90 gallon. If you are growing them out as juvies, then I would recommend getting 6 or 7. They will pick off the weakest of the bunch and you'll be left with the top specimens.
Glad it wasn't just me that was a little surprised by the goldfish idea!
Thanks for everyone's input, I am just kicking around ideas at the moment as it will be a few weeks till the new house is painted etc so I can set it up. But I have never kept piranha and love the idea, basically it's coming down to either piranha or a pair of snakeheads