Gambusia are in the family Poeciliidae as are Poecilia and they are both in the subfamily Poeciliinae,
this makes me think that the genus on one of the fish is wrongly classified and a hybrid is very possible.
I'd still like to see scientific proof though.
The classification is not important as some species are mis-named or just changed by science for some reason some genuses are split but it's a long job.
The scientists studied the endlers for over 30 years (1975-2006) before finally describing the as a new species. Many people before this classed them as guppies as they are easily crossed with all lines of guppies, but until they started using genetic make up of the fish they discovered the genetic differences to classify them as a new species.
The primary way to differentiate species are.
Lateral line count
Lateral line to highest scale count
Lateral line to lowest scale count
Dorsal fin ray count
Caudal fin ray count
Gill raker count
Extra with livebearers
Gonopodium structure.
With livebearers the primary way to place fish into a genus is by the gonopodium. The structures is firstly grouped together to form the genus this is why platies and swordtail's are in the same genus and not two different genuses.
This is a typical shape of a gonopodium of a
Xiphophours
And now two different species of
Gambusia. (primary differece is the gonopodium is pointed in gambusia, classing them closer the the
Brachyraphis than than
Poecilia)
Gonopodia structure of
Poecilia reticulata in Fig a.
Poecilia winge, in Fig b and c
As u can see the structure with in the same species can be slightly different but on the basics it's the same design.
Anyway I've rambled on here but think I've explained the basics now and crossing between different species is difficult as the structures are different and makes fertilisation very difficult with out artificial insemination being used.