Ok i have read and read about this and this is the mist reliable summary i can come up with:
Endlers/guppies/mollies can cross breed with each other yes. They are all poecilidaes.
Swordtails/platys can cross breed with each other too, they are xiphophorus.
Under normal circumstances xiphophorus and poecilidaes CANNOT cross breed. Under scientific conditions, however, they can all be artificially inseminated and by controlled conditioning can then give birth to cross breeds.
Simply, a xiphophorus gonopodium does not fit a poecilidae, and vice-versa. So left in the tank will not provide off spring but may engage in 'courting' behaviour.