While Poecilia species hybrids are possible, the further apart they are, the less likely they are to produce. Beyond gnetics, it is the interaction, the gonopodium structure. Then there is the genetics.
Mollies, not being from the same general area as the guppy, and guppy like species, would have a difficult time, at best--that's why there haven't been any guppy/molly hybrids.
There are few natural hybrids in the wild with livebearers. Few in the fishroom either; a lot of conditions have to be met for them to work. For example, crossing the currently popular Endlers to a guppy is not a sure bet--though they look similar and come from the same genreal geographical area. Some people have had success; some didn't, in trying to 'cross them'--whether fancy varieties of guppy or wild strains.
But a guppy & molly--naw, ain't gonna happen. But the molly that is called a guppy tail molly is just a genetic variation of the tail, caused by a genetic anamaly.
Actually, Guppy-Molly hybrids have happened. They have been known since at least the 1950s or 60s, as there is a photo of a hybrid male in the Innes' 19th edition. It is a rare but well documented cross and is apparently sterile. A google search will find numerous photos and reports, many of which are from knowledgeable aquarists.
However, the Guppytail Molly is a fish with a mutation that produces a delta shaped veil tail, like a guppy, but not via hybridization with guppies. That misconception was the start of this entire thread.
As you may already know or maybe you don't care, I have focused my efforts on Fancy Platies & Swordtails, so I usually don't have much to say on other topics. However I do know that there are/ were two types of "Veiltail" Mollies. The "Wood Veiltail" produced by John Wood and made "famous" by Dr. Joanne Norton and the "Guppytail" Molly out of Asia. The Wood Veiltail had a wide triangle type tail and the Guppytail usually has an asymemetrical tail being longer on the bottom with rounded edges. This is the type which is still available occasionally. Most believe that the Wood Veiltail was gone shortly after Dr. Norton stopped working with fish after becoming ill. Even though Dr. Norton made these fish available to ALA members, no one kept the strain going.
I have been raising guppytail mollies for several years. They are a molly not a hybred unless it was done 50 years ago. Of course we know Jo ann Norton produced along with a few other people the famous veiltail molly. These are probably a mutation from their stock. The major difference is that the guppytail has a longer and always rounded tail. I believe this fish originated from some of this stock. I have one tank alone with over 100 junior guppytail mollies and they come in a wide variety of colors but frequently throwback to a wild green molly color. This fish has been getting a lot of talk lately and it in my book is a true molly. A beauty that is woth every penny to add to your stock. YThey do best in a well established tank. I have never added salt to my tank or had to steal away the fry. My mollies never eat their fry. I feed them frequently and have prolific and large fish. email@example.com
Dr. Joanne Norton's Veiltail Molly stock came from Steve Sauders who got his stock from John Wood "Wood Veil". Veiltail never mutated in her stock, but she developed many strains using different Wild forms of Molly to breed into the "Wood Veil" which is why the strain is associated with her and not with Mr.Wood. If you don't believe me, check her articles or contact her friend (and mine), Mr. Glenn Y. Takeshita or his articles on this subject. If you back up and read my other post you'll see that I mentioned the rounded edge on the Asian Veil Molly which is what's out there now (since it is believe that the "Wood Veil" was lost shortly after Dr. Norton had to stop keeping fish due to illness).
We should just start calling the guppytail molly a paddle tail which is what it was once called. That title gives a definite description of it shape and removes the misleading title. firstname.lastname@example.org