Well, you seem to have some probable answers! I ended up having some fry come out like that, but my mollies were never near guppies or platy or any other fish they can interbreed with like that(Unless they can interbreed with rasboras).
In anycase, I have a few of them that look exactly like that, the brood came out mized, so I have some liek your others as well(I put a video of them in the video section, you might be able to make a few out). They are Gold Dust Molly crossed with Silver Mollies. =) Mine are anyway. I also saw an adult one at the petshop today, it looked like a more brownish Silver Sailfin Molly. Maybe that's what you have.
You can see a few in here, if this helps, some of the light brown ones, and also the ones that are kinda clear and orange are from the same brood. The black and gold, and the Silver ones are from different parents. Fullscreen it if it doesn't work right. lol
[ame="http://www.youtube.com/watch?v=etvDSINnx1Q&feature=plcp"]My fry(Baby mollies) mobbing at the Algae Wafer - YouTube[/ame]