Breeding mollies(tropical fish)?

I have a 50 g tank with a dalmatian molly and a white molly , and some other fish. My dalmatian molly got "pregnant" and popped out 21 baby mollies. I have them in a breeding box and have special first bites fish food for babies and am planning on selling them/ giving them away if i cant sell them. I do not want anymore fish, but I think that this could be a good money maker for a 14 year old. (I have a real job but this could make me some money and I enjoy doing this.) If I keep some fish and they, you know will they be demented because there related? I want to know because if I have tons of fish breeding I will have all sorts of babies and this could become a real job. is this a good future plan? as a back up