As a rough guide my turtles eat fish 3x their size
I've mixed cichlids and a turtle, however not any of the lake malawis (eg demasonis, electric yellows) or frontosas, because they require lots of rocks and caves which is bad news for a turtle.
Yep and it's easy for them to get caught in holes and drown. They're not the smartest of creatures when it comes to thinking "yeah I can fit through that ... "