I had one before. They're pretty cool. I'm assuming you're referring to the Pelomedusa group. These guys are more in habit like our US mud/musk turtles in that they do not swim a whole lot, so depth isn't a huge issue. So it would have to be a large tank with varying depths to accomodate a African sideneck and a large cichlid.
Mine was about 6 inches and I never kept fish with her, but she was pretty aggressive to other turtles. So my initial suggestion is to keep these turtles alone.