Because guppy males try to mate with something doesn't mean it is going to happen. Who knows what  will happen in this crazy world. But I would bet the Guppy x swordtail cross is quire unlikely.

In nature female guppies have a fair amount of choice as to who they will mate with (except for the occasional "sneaker"). Studies have indicated they will often select three different males. (Perhaps the genetic diversity allows some of the 5-20 fry to survive.)

Female guppies also reject males, which are proved to be diseased. They seem to know if a male is infected with gill flukes for instance. They are most attracted to the more colorful males. Orange and black, perhaps because of their contrast is often a winning combination. Females also seem to prefer the more energetic types and so the best dancer often gets his girl. ;)

Tangentially, a study of wild swordtails also indicated that females showed preference in who it mated with and even kept company with. A study published in Copeia several year back suggested that in some cases a wild X. helleri female would keep company with a large male in part just so he would chase the pesky smaller males away. In that arrangement he gets to mate with her and she has more time for foraging and spends less energy (=food) avoiding the sneaker males. That may mean that she will drop more fry.

The energetic display and coloration of male guppies cuts two ways. Those males may mate with more females, but they also make themselves more visible to more predators. There have been a whole clutch of studies indicating that in waters with large and efficient predators (usually pike cichlids, various species of Crenicichla) the colorful guppy males get picked off so fast that the entire guppy population has males which average must less coloration than in those of (often upstream) guppies which face mostly small predators which can not take an adult guppy male, but only fry.

If females can't avoid persistent males, they may forcefully drive undesired males off. They have been known to attack them and even remove part of the gonopodium.

Livebearer species are distinguished from one another partly by the shape of the gonopodium. They develop quite different patterns of hooks and spurs and under magnification look a bit like mutant can openers. Females of each species are designed to accept the specific pattern of gonopodium characteristic of their species.

A Genus is used by scientists to group species, which they think are closely related. Similarities of form, including gonopodiums, are considered in the analysis. There is enough argument these days over what even constitutes a whole species. The idea of a genus is a human concept and, especially when looking back on fish classification or taxonomy, sometimes pretty arbitrary.

However if the classification is done "right", theoretically females of one species in a genus have similar reproductive organs and might be more likely to interbreed with males of closely related species that with males of less closely related species.

There are a few rare instances of mollie x guppy crosses. Until today I have seen only two photos in about 75 years of aquarium literature. Ah, there is another one in the Aqualog book on livebearers. (See below.) In fairness to those who feel that there have been more guppy x mollie or mollie x guppy crosses produced than we realize, it may be that people have just not noticed another batch of gray fry. What the odds are of them growing up is likely less than that of mollie or guppy fry, but maybe it isn't as rare as I think.

The hobby mollies are not pure strains most of the time anyway, unless recently collected. According to Lothar Wischnath in 1989, three of the Poecilia (formerly Mollienesia) - sphenops, latippina and petensis contributed towards the early domestic mollie strains. More recently velifera was introduced to develop the high-finned strains.

If J-ME's mollies drop, the first question I would ask of them is have they dropped a half-dozen times since they were last with a male mollie. If they haven't been with a male mollie since last July, then I would look more closely at fry as possible crosses.

If you are looking for images of crosses, the old Innes book published a picture of a guppy x mollie cross. In Wischnath's Encyclopedia of Livebearers (TFH 1989) on page 274 there is a P. reticulata x P latipinna cross which looks like a gray mollie with guppy-like fins. Aqualog's All Livebearers and Halfbeaks, edited by Kempkes and Schafer, has a color image on the right bottom corner of page 167. There are faint colors in the fins, which echo what one might find in a male guppy.

Interlibrary loans could inexpensively secure you the use of those books. Innes's Exotic Aquarium Fishes (William Thornton Innes) is often available through used book sources. I would try for an edition before TFH got the publication rights. That one is probably in many libraries.

Platys (originally called platyfishes) are more or less Xiphophorus macularius, the swordtail most often ancestral to the hobby fish is largely Xiphophorus helleri and the variatus is in good part X. variatus. Even here, researchers like Dr. Myron Gordon, who were trying to cross strains and species of those fish, found crosses originally somewhat unusual and difficult to get. They discovered that certain crosses would produce cells, which would develop cancers. (That is also very common in the eggs and fry of crossed killifish species. Some cross spawnings can not even fertilize an egg. In other cases eggs or fry die. Those are all "isolating" mechanisms.)

Today, according to the Xiphophorus Stock Center, their "Xiphophorus hybrids are almost always fertile and are extremely valuable for their genetic variability and their very specific susceptibilities for many different cancers." They sell them to many universities and research facilities. Interestingly, they mention that requests for live stock may be addressed to the Center's director. I'm assuming that a hobbyist must demonstrate that they are pretty serious about keeping the strain pure and that the Center must have extra specimens of that species and strain available before their stock would be sold to an aquarist. They are in need of additional operating funds and are building an endowment fund in case anyone has extra money they would like to donate. (I'd ask about a tax credit for a deduction.)