Ive heard muppys are a thing and totally possible. I have not actually seen any yet and heard its pretty rare. I do have mollys and guppies in the same tank so if I end up with muppys at some point ill send pics lol.
If you're talking about a golly or muppy, the phenotype differ per cross. Guppies come in different colors and shapes. But so are mollies. Typical for most molly fry is that the head is a bit more larger than the rear of the body. While guppy fry are slender. But there are also slender molly species. Fry of those are also slender. Which means that fry of a cross between the 2 species can differ as well.
And the chance that they'll crossbreed is if you keep 1 specimen of the opposite sex of each species in 1 tank. Keeping a number of both species in 1 tank wil hardly till none result in a crossbreeding.