Platys, swordtails, variatus and other members of the genus Xiphophorus have been sometimes crossed. Indeed, through a reseach laboratory started by people like Dr. Myron Gordon (if you want someone to Google), it was discovered that even with the Xiphophorus many crosses would develop cancer and they were used to study malignant tumors.

And Xiphophorus males of different species do have similar shaped gonopodiums. (One of the way "they" separate Poeciliids into species is by the shape of the gonopodium. Female livebearers also have genital vent which is shaped to take the gonopodium of her species and to exclude males of other species. SOMETIMES crosses between Xiphophorus species do happen, but even there crosses don't always happen.

J.J. Scheel did a lot of crossing experiments on African killifish in the 1950s, 60s and 70s. Many of the crossed individuals didn't even develop a fertilized embryo. Among the embryos which fertilize, a lot of them died in the egg. Various cancers again were cited as prominent causes.

Where different killie species did produce crosses, they were often (sterile) "mules, either in the first, or second or third generations. The ability to cross has been proposed as a way to see if different species are closely related. People have been surprised sometimes by who will cross with whom that way. DNA studies later have sometimes demonstrated that killies able to cross were more closely related than was first assumed. Skeletal structures, especially jaws, seem to also have been used in these studies. There are some who have crossed and while they are within the same genus, are not as close as some people assumed.

American minnows, sometimes of several species will sometimes spawn at one time (over three-foot wide nests constructed by larger minnows) for instance and some crosses will result. Don't know if they ever produce another generation or if because they are a little different, that their role is as prey for predators who then leave more of the parent species alone.

We've talked about this several times before on Guppylog and I'm sure some thoughts have been left out. (Maybe Google [Platy molly cross Guppylog]. In 100 years of keeping platys and mollies in the aquarium hobby, I don't know if any fry from such a proposed cross have even been found. If you Google Poeciliid DNA studies you will get a lot of taxonomic and systematics stuff. (Borrow a scientific dictionary from you library first.) Certainly if you Google platys mollie crosses, you will get a lot of discussions but not much evidence of

I applaud your active and creative imagination. It is an interesting question. Great things have been accomplished by people who remarked "I wonder" ("I wonder if infections can be cured" or "I wonder if people will be able to almost instantaneously communicate with others around the world.")

Still, just because a male live bearer tries to mate with another species, that doesn't mean that they succeeded. I have seen very, uh, urgent guppy males chase around a plant and accost (egg laying) Corys and killifish. The response of the egg layers (sometimes also male) was often one of considerable surprise. I don't think that the world will be viewing any "gories" or "gullifish" soon. While less improbably, I don't know if the world will soon see any mollatys or plallies soon either. :)