Platies and guppies can't interbreed and that's definitely a platy
I don't know why you're so sure that this is a platy? For it's definitely a molly.
Wel, livebearers which impregnate themselves is something I haven't witnessed myself in way over 40 years I'm keeping and breeding them myself.
Sex change with livebearers is a fact! I've witnessed that quite often throughout the years. But only females can change into males and not the other way around. These transgender females will keep the gravid spot. That wil not dissappear. And once their anal fin has changed into a gonopodium, they are able to fertilize other females. I do have to say that scientists will most likely reject what I'm saying here. For till this very day, it's still not scientifically proven that this is possible. But I know it's a fact that females can change into males. And yes, in general it does happen when the ratio male/female is out of balance. It doesn't even happen with livebearing toothcarps. It also happens with other species. And it's a way to preserve the species. "Nature will look after herself"...
It's different from the phenomen "late males". Late males are males which look quite similar to females when it comes to body features but show their masculine features at an older age. Which results in large fish to look at.
Mollies can absolutely interbreed with guppies. It's already been mentioned overhere. they're of the same genus "Poecilidae". I had a crossbreeding once but it didn't lok like the picture shown. Hybrids of those two will look more slender. They're not exactly beautiful fish to watch in my opinion.
But eventhough they're able to interbreed, it's not very likely that it will happen when you put those two together. If both genders of both species will be put in one tank, than they'll choose their own species to mate with. A crossbreeding hapens faster when one of them doesn't have the opposite gender around.