Guppies and mollies usually cohabitate well together.
They can crossbreed if they aren't kept stocked with enough breeding mates of their own species. So I would suggest:
2 male guppies
6 female guppies
2 male mollies
4-6 female mollies
You could still do 3 cories or 6 kuhlis (sand substrate for kuhlis if possible) and be fine.
If you wanted an all male tank so you didn't worry about fry (although with both species you should get very few if any fry survive their parents and other fish eatting them) you could do:
8 male guppies
6 male mollies
3 cories or 6 kuhlis
(I wouldn't suggest this setup as much, simply because I'm sure it would cause a slight bit more aggression between the males not having females to mate with, as well as not giving them as close to a natural setup as possible.)
Hope that helped.
