Here's something I read on the Wikipedia entry for X. maculatus - "The fish commonly sold in pet shops is not a pure strain of X. maculatus, but is a hybrid between X. hellerI and X. maculatus. In general if the male has a sword shaped tail they are called swordtails. Otherwise they are labeled platy. Color and fin shape vary wildly in the aquarium trade."
So, apparently, platys and swordtails can hybridize. I read some other sources, not as reputable as Wikipedia (if that's possible) that other livebearers can interbreed too.