Yeah, I've worried about that too...I think that's why the dwarf/ mata mata enclosure works so well at the Toronto Zoo. Matas don't bite anything the can't swallow, and have very weak jaws regardless.
when my caiman got bigger he tryed to eat my turtle but they didnt have a healthy co existence the turtle use to try and eat what ever part of the caiman was by him till i sepparated them ive seen it done it just didnt work for me watch them carefully
I've done it and have seen it. Sometimes it works, sometimes it doesn't. It really depends a lot on the croc/gators you use. A 3 foot croc/gator can crack a shell like eating a candy bar.
It depends on the species, turtles that live in close proximity to crocdilians tend to have more robust shells...example cooters vs sliders, or african muds vs african helmeteds...
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.