Only breed male guppies to female mollies. If you breed a male molly to a female guppy, the size of the fry will kill her. I have seen some very interesting muppies. I think they are cute. I understand that people want to keep things pure but many animals are crossed to get today's pets. As long as you are honest that the fish were a mix and do not try to sell or give them away as pure breed I see no problem with it. If they can breed than they are closely related anyway. I only have a problem when genetic manipulations get involved. Nature purposely limits itself and it scares me what can of worms scientist are opening.