The issue with livebearers is that males are always ( it seems) looking for a female of any fish to mate with even tho the mating is usually unfruitful. It is more common for Guppies to breed with Mollies ( both ways) and Swordtails breeding with Platies ( also both ways) since those two groups are related more closely. With that said, Platies do have the ability to hold sperm so the platy may have been pregnant the whole time? In your pic, the bottom gold colored fish looks more a misshapened molly than a " typical" platy so you may in fact have a pair of mollies and not the mix you believe you have. The real answer will come when that fry grows. If it was a mix, it should turn gold in color ( based on the parent's colors). If it stays black/marbled, I would think more along the lines that it's from a previous pregnancy. If it turns out to stay marbled and is a Molly, that should tell you that your female is most likely a molly and not a platy. IMO
Hope this helps.