Mbuna, Hap, or Peacock??? "Malawi Cichlid" refers to around 1200 species...
Its fairly possible, Im not sure about the species of turtle, I'm in no way a reptile expert, but I know of the Native Softshells where im from get pretty large, to where they would need a 300g+ aquarium to be able to swim freely, I caught one fishing that was about 14'' of shell, not to mention they do eat fish.
Malawi Cichlids though are fairly fast swimmers, and also because of their territorial nature they are always alert when "sleeping" so it would be hard for the turtle to catch
Another issue you'd have to deal with is waste, turtles produce A LOT of waste, which can cause Ammonia to spike killing most if not all of the fish.
So... with all of the cautions noted, If I were doing this, I would go with the more aggressive species of Mbuna, your M.Auratus, Ps. Demasoni, and/or Ps. Elongatus.
Here's why, Mbuna are more tolarant to water conditions than your peacocks and haps, Mbuna are Rock/Cave Dwellers so with enough rock work in the tank they would have a safe area to go, AND with the more aggressive species they tend to stress a little less than your more docile species when being aggitated by an aggressor (I.E. Turtle)
I would look in the future to upgrading everything for this to work, I'd say a minimum of a 125g, with a 40g wet/dry trickle filter, possibly even higher to compensate for the turtles wastes
Good luck
