The Pseudemys species (cooters and redbellies) are known to interbreed with one another. Probably the different Trachemys species can as well.

Are you seeking to create hybrids? If so, I suggest you reconsider. There is no need to create the confusion in the turtle trade that already exists in the snake trade; keep those bloodlines pure and everything simple.

If you want to produce some different animals from what everyone else has, why not try captive-breeding species that are just uncommon in the trade?