In my LFS, I found a platy called dwarf platy which size is only 1.5". Much less regular platy 2.5".
So can they mate with regular platies like red wag tail ?
Though I have never experienced this myself, I would have to say that the answer is yes. I've never even heard of a dwarf platy, but it most likely just the result of selective breeding of regular platys anyway. I have also heard of livebearers mating with different types(ie:Swordtails mating with platys). Those livebearers will mate with anything.
My bet is YES. A dwarf platy female might have some problems giving birth to fry from a full-size male, but otherwise I don't see any reason why they couldn't breed.
I've heard of it, but I don't believe it. Mollies are only passingly compatible with guppies in the same genus; I can't imagine them crossing with fish from a different genus altogether.
No such species as a Dwarf Platy. Many wholesalers and retailers name their livestock rather creatively. Like "Teacup" Stingrays that turn into serving platters, and Dwarf Albino Frogs. Just because it's small now doesn't make it a dwarf. But to answer the real question all of your commonly available members of the Platy's genus, Xiphphorous, hybridize indiscriminately. Odds are it's a hybrid to begin with.