I have recently read abook that says artificial forms of the Molly such as Balloon .Lyer tailed and veil tailed are harder to breed and keep.Is this true,as I have got a female lyer tail who has just given me 15 babys and both her and the fry are fine.The book mentioned that the natural forms (sailfin and Black mollies )are more resistant to changes in the water such as temperatue or salt levels.
Can any one shed sone light on this ?

The balloon mollies, lyretail mollies, etc. are mutations. In the wild, such mollies would be slower and weaker and more likely would be eaten by predators and would not survive to breed themselves. Balloon mollies have some of their organs smushed up together and have trouble swimming, both of which can make them more apt to die from any cause. Often when a fish has a mutation that gives it a different shape or fancier finnage, there are other deformities that go along with that that are internal so you don't see them.