I dont know if I would even put a molly in a 10 gallon for long term...they get to 3 inches...and can actually be a pretty good bully.
Yes, mollies and guppies can crossbreed muppies...which the only thing I had heard about muppies is that they are sterile and cannot reproduce.
I have never dealt with female guppies, I have always kept males (purposely trying to avoid fry of any kind). With the male guppies, I find the larger the group the better as they like to get into small arguments, and having at least 3 can spread it around...I have 9, and it is very entertaining to watch them break into their little argument groups, nose to nose, swimming in a circle together while they bicker, and then swim about their way after. I have never kept less than 4 guppies together though. If you go with guppies, stick to the fancies as their size is better fit for a 10 gallon...if you get the specialty guppies, like Sunrise Tequila Delta Guppies, then you can end up with a much larger fish measuring at 2 inches full grown...trust me, there is a BIG size difference between a fancy and a delta!!! Fancies also seem to be calmer than the others.
I cant comment on platys, I dont have them, have never had them, and probably will never have them. They just arent my thing I guess. But I know they are great fish for a 10 gallon.
If you are worried about breeding...stick with all males or all females...or do all male guppies, and either all male or all female platys. (Male guppies wont bother many other fish at all, however, they will happily aggrivate to death a female molly or female guppy.)