Wether it's desirable to crossbreed two different species, is so personal. We can argue about it or we don't. But just know that a lot of aquarium strains of whatever species are the result of crossbreeding or just simple linebreeding. Linebreeding can be done in wild specimens and crossbred specimens.
For example, any Poecilia can breed with any Poecilia :
Poecilia "uuppy" reticulata x Poecilia wingei
Poecilia "guppy" tericulata x Poecilia "molly" sphenops or velifera
In most cases fry is infertile.
Correction:
Every Poecilia species can not breed with a random other Poecilia species. A number can but not all. It's not spefiically the genus that separates them from crossbreeding. It's the shape of the genitals that makes it possible or not. And of course, endemic locality can be also a reason why they won't cross with another Poecilia species in free nature.
So, short: The sexual organs need to be compatible. Every male of an ovoviviparous livebearer species within the Poecilidae family (note: The Poeciliidae family is not the same as the Poecilia genus. But a part of the Poeciliidae family). The gonopodium of each individual ovoviviparous livebearer male, is differently shaped. The female sexual organ is shaped for that specific gonopodium shape of a male of her own kind. Just certain related species can be compatible like already mentioned.
It doesn't always mean that the offspring will be infertile. They can be fertile buit their fertility can also decrease till even zero. That depends on each individual offspring.
To use the examples of Avel1896:
All guppies (Poecilia reticulata, Poecilia obscura and Poecilia kempkesi) can cross with all endlers (Poecilia wingei), all Micropoecilia species, all Limia species and all Molly species (P.sphenops and P.velifera are just two of them for there are way more wild molly species than these two). But they can not cross with the Poecilia formosa (also a wild molly). You do read three different wild guppy species that I've mentioned. For not all guppies are derived from Poecilia reticulata. And not all wild guppies are Poecilia reticulata. Nowadays, Poecilia wingei (endler) is also in the same summary as those wild guppies as long as we call them endler guppy instead of endler. Despite of carrying different DNA than a guppy, they are very close related.
Mollies and Limia species can crossbreed with fertile offspring.
Different species in the same genus can sometimes produce offspring hybrids which are sometimes sterile. It's unlikely that the two you mention are breeding, but perhaps going through 'pre-progammed' motions.
You're correct...
yep!
and they think it succeded becuase the guppies and mosquitofhs were already impregnated
Well, that's mostly the problem. Lots of people just don't know that females of ovoviviparous livebearers can store sperm packets for over a year. And then when they put two species together and fry occur, that it must be a cross between those two species.
yep, guppies would probably try to mate with a gambusia and other way around
So, true... they'll try but no actual mating will happen...
I have Mosquito fish/guppy fish hybrid
Mom was Mosquito and dad was a number of different fancy males
I have only purchased male guppies and female mosquitoes and this is what I got
They can not crossbreed for the simple reason that their sexual organs are not compatible. This means that those fry were from a female that had already stored sperm packets and she used them to fertilize her eggs. But an actual mating between these two species is technically impossible. Unless they have artificial fertilization. So, with the help of man.
There's a good chance endler's are gone in nature, and they are pretty close to gone in the hobby
Finding wild endlers from the Lake Campoma isn't possible anymore for that lake has been dredged. Wild endlers from the Cumana region can still be found in the wild. And pure endlers are still massively present in the hobby on this globe. But mostly at serious endler keepers/breeders. So, not specifically the commercial world. I'm one of those keepers/breeders that also keep and breed N-class endlers.