I can tell you absolutely that it is NOT possible for a paddletail and CFB to interbreed. In terms of biology, they are more distantly related to each other than humans are to chimpanzees, for example. Furthermore, due to the aggressive behavior of paddletails, no responsible pet keeper would keep these two in the same tank.
The old-fashioned definition of the word "species" was two groups of animals that cannot interbreed. There are some exceptions, where two species in the same genus can interbreed (horses and donkeys, for example). Paddletail and CFB are not even in the same genus (Cynops and Pachytriton). It just doesn't happen, and cannot happen.