As far as piranhas go, you could probably go with Serrasalmus Rhombeus "Black Piranha" but do keep in mind they need to be kept alone. If you can get past the whole needing to have more than one fish thing, they are a perfect match for what you are doing they grow incredibly slow and require a large aqaurium when older. A 4"-5" specimen would be ok in a 55g for quite a while. Estimated mature size is around 19"-24" biggest I've seen in captivity was around 17" single piranhas do tend to get a little boring after a while, but given the circumstances you're pretty limited in your choices. Most of the fish that will be fine in a 200g - 300g won't do very well in a 55g, not even as a juvenile specimen.