You can not cross a molly and a swordtail as they are in different genus. Molly and guppy cross is possible, but uncommon and normally sterile. Swordtail and platy cross is common and often used in the hobby to create strains. These offspring are viable. Generally though in this hobby hybrids are looked down on and are worth less than any well bred pure strain. I saw some platy x sword hybrids at a auction and there was nothing fancy about them. Typical platy body with a short stubby sword on the males. Boring fish IMO. They sold for slightly more than a bag of mixed common livebearers would. All the pure 'show' strain guppies and all they wild type livebears blow them out of the water.
Dogs are a single species that has many different varieties. Crossbreeding dogs is completely different. That would be like crossing a lyertail golden molly with a black shortfin molly. Same species different strains/variations. Crossing platy and sword would be equivalent to a wolf-dog hybrid in the dog world and a bengal and ocicat in the cat world. It is the same with them, some people support their creation, while others are opposed to doing such things.
We already have sailfin and lyertail crossed swordtails, platies, and mollies. Depending on the species: Do to the length of the reproductive fin, these fish cannot breed. All most all of these man made fish have reduced reproductive rates. Here
is a video of a really fancy swordtail I use to own. Best way to describe her was a red-eye red, sailfin-lyertail swordtail fish.... or something like that. Sure she was pretty and all, this particular strain cannot reproduce do too all the finnage. They also bring lots of newer issues to the table, like they need low current tanks. Also their fins love to tear. She hopped back and forth between the bare bottom tank and a regular tank. Her health would decline in a normal tank, she only did well in the bare bottom tank. I liked this fish a lot, it got to the point were I was considering knocking her out and cutting her fins back. I lost her before I felt safe to attempt such a thing.
I am not against the man made common livebears, there are a number of cool strains. However wild strains hold much more of a appeal for me now. Some wild swordtails have a super long sword.