Naturally, as suggested by others, making sure the turtles needs are met is a must, such as providing a basking spot with the proper lighting, if the species you choose needs such. Of course you should do the research and not make assumptions prior to moving beyond this…
I have a close friend that works in a pet shop (Pineville Pets) which has a 300 gal ‘pond’ that contains 3 adult RES, a handful of African Cichlids, a few convicts and some Goldfish… The RES catch fish on occasions (1 every months or so)… mostly the goldfish, but he says they catch Cichlids once in a while, I’ve seen one of them eating a Convict during one of my frequent visits…
If keeping a large aggressive Cichlid such as an adult Red Terror or a Trimac with a small turtle, I would be concerned the fish may chew off a foot or at least cause an unhealthy amount of stress to the turtle…
If the turtle is an adult and the Cichlids are small, I would be concerned that the turtle would eat the fish…
Naturally there can be a balance with the right turtle, the right fish and the right set up… but it is not something I would expect your average Joe to be able to slap together and it work long term…
Miguel, don’t get upset that people didn’t accept your experience as the ultimate answer to the question. Your turtles are still young and have a lot of growing to do. Your experience may (or may not) be very different a year or two from now.