Lissemys punctata isnt a species i've ever seen available here, very nice find!
I would not mix it with fish, even larger ones are likely to fall prey to it especially slow moving bottom dwellers. Might do ok with fish if they are both larger, and fast moving top dwellers, as this will make them far less likly to be eaten, but all it takes is one lucky shot and a nice chunk of fish and the turtle will be hooked and hunt down the bigger fish as well!