I know some people do it, but I would advise against it. I tried mixing a red eared slider and an Oscar before, and the Oscar terrorized the RES. They were about the same size and the Oscar would constantly attack the turtle. Plus the bioload of the turtle will be a lot. You will need a big tank to house large cichlids and a turtle. Plus most turtles require a place to dry off and bask under a light. This takes away from fish room (decrease territory space for the large cichlids) which can cause aggression.