Here are three sources I found about keeping turtles with caimans:
"It is not uncommon to hear reports of turtles and even fish nibbling away at a caiman's tail or toes, especially when they don't have sufficient food themselves. However, some owners have reported that caimans get along well with turtles, as long as the turtle is too big to fit inside the caiman's jaws. An adult caiman is quite capable of crushing a smaller turtle's shell." - posted on
www.crocodilian.com
"I've seen dwar caimen kept and housed with Large Belize and Nic Sliders... I've seen crocs housed with large Podoc turtles, alligators with sliders and cooters... Aslong as they're bigger than the croc I dont think you should have any issues." - posted by member Shells-N-Scales on MFK
"Enclosure mates can include but not limited to other caimans of similar size(if there is sufficient space) turtles(painted, sliders etc but no aggressive turtles such as snappers) and sometimes fish. These are likely to be eaten so don't put your favorite fish in with them. Smaller fish have a good chance at being left alone when the caiman is larger. " - posted by member Lepisosteus Platyrhinchus on MFK (note that LP owns Claudia, a female dwarf caiman and has a 126 page build thread for her enclosure, check it out it's awesome)