guppy and molly breeding?

yes it is possible since they are in the same genus Poeceliadae

I may have spelled that wrong but at any rate the off spring would be sterile
 
Yes it can and will happen, and you will get viable fry out of the mix. The fry won't be worth anything really since they are hybrids, though I suspect many of the mollies now available in petstores are not 100% molly genetics, and have been bred with guppies to attain some of the colors. The Guppies and mollies don't care if they breed with one another, as they don't see a difference.
 
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