Crossbreeding to take place, you have at closely related DNA. The mollies and platys are closely related and thus will crossbreeding given certain conditions.
But neither mollies nor platys are anyway related to corys and thus the chances of crossbreeding are absolutely zero.
There are numerous example of crossbreeding in the animal science such as liger (tiger and lion), wolphin (dolphin and whale) and grozlar bear (grizzly and polar) ... just to metion a few,
In fish, the best example is blood parrot. Cross between midas and red devil cichlids.