Shark, IMHO I would say that mollys are going to be a little easier than guppys, most guppys have been inbred so much that they are very fast to develop problems, not saying mollys havent but they dont seem to be as bad. There are alot of sites out there that talk about the inbreeding to get fantails and certain colors and what have you to make the general fish hobbiest happy. I would say this, if you are serious about breeding your fish you get take the time and make the investment of some quality fish. There are quality breeders out there and they will get you good fish......Much better than your LFS. With that you are going to pay more money for them but I dont think you will get hurt in the long run
Another thing is do your research online, read everything you can on raising and breeding fish. Each species requires and has different things to promote breeding habits. Learn them and you will be well on your way to being a top notch fish breeder, what ever fish you decide
HTH
Les