I did a little googling and found these excerpts:
"Yes! I have pics! taken today with the new 'fish' setting i found on my camera! let me know what you think, beacuse i am so sure she's half half, ive never had her mother (a molly) in with any male mollies, only guppies. im sure of it. her name is baby, (a name that stuck) she is about, well, nearly a year now i think x
WOW i have always pondered about my cross fish, but i searched google for details and found this, is this TRUE??!! if so, wow!!! i neve knew it! maybe this is what happened??? bizarre eh?
'Female mollies have the amazing ability to become pregnant without a male molly.
All in takes is stimulation by a male fish of any other similar species like guppies, swordtails or platties. The male is not the father of the fry and the babies are not cross-breeds. Penetration by the male simply serves to stimulate the female into producing young that are effectively clones of their mother.
This process is called parthenogenesis and is found in many insects and some frogs as well as mollies."
"A mollie can store sperm from a male for up to 5 sets of fry. So don't think she is done having little ones here for a while. Guppies and mollies both are livebearers and classified under the same family Poeciliidae (containing more than 300 species of fish). While there are possiablities of cross breeding it isn't likely and those fish that are crossed are known as mules, since they rarely live to adulthood and are incapable of reproducing. "
"I disagree, mollies and guppies are different species and do not produce offspring. I kept fish for 40+ years and never had those two cross breed, even though they were kept together. It would be like trying to breed chickens with ducks. Doesn't work."
"Mollies and Guppies are both Poecilia. I'm assuming the above posters know nothing about the subject or otherwise would have refrained from making those comments. Yes, you can cross the two species. If you cross a male molly with a female guppy you will kill your guppy. If you cross a female molly with a male guppy you MIGHT get fry and some MIGHT survive and they MIGHT not have the genetic defects common in such a cross but you won't EVER be able to produce viable offspring from the crosses themselves as they are essentially mules, and those that have been successfully bred have never survived to produce a viable breeding population which is why you don't see Gollies in the stores everywhere. They come out deformed and have a lot of genetic problems.
If you care about your fish not harassing each other do yourself a favor and either start a separate tank for the Mollies, or just get rid of them and concentrate on Guppies, (or vice versa).
If you can't do that, at the very least get a breeding net/ tank which floats in the main tank and keeps the mom fish separate from the rest of the fish.
Either way DON'T add any more fish until your tank is cycled!
Here is a picture of a Gollie."