will swordtails and mollies cross breed and how likely is this

i have two female mollies one male,one sword tail male and female,in 20 gal tall tank,

Both Palties, guppies and swordtails all belong to the same family of fishes, Poeciliidae. Thus their genetic make is simillar and not identical. So, cross bredding is possible except their offsprings would be sterile. That is unable to repoduce themselves.

FROM http://www.aquariacentral.com/forums/pri.

Although platies and swordtails will breed in captivity, their propagation tends to be a bit trickier than most livebearers. Male swordtails are distinguishable from females by both their gonopodium and sword projection on the tail fin. Do not be fooled, however - some males may go their entire lives without ever growing a pronounced sword. Reliable sexing is attained only through the absence or presence of the gonopodium. The females of both platies and swordtails also exhibit a darkened patch toward their vent, which expands or contracts based on her degree of pregnancy; very pregnant fish will exhibit a very large dark patch.

These fish tend to mate with just about anything they can get a hold on and can easily hybridize, thereby rendering the offspring infertile. This occurs when too many generations of the same bloodline continue breeding. When successful broods are produced, the fry are quite fragile and may be physically stunted if not given a sufficient amount of a proper diet.

Not the most prolific of the livebearers, mollies tend to brood every eight to 10 weeks (instead of four), but may birth as many as 130 to 150 fry. Just remember to feed your mollies a hefty amount of vegetable matter if you want them to breed. Although it is not fully understood, the reproductive chemistry of these fish will not properly operate without a plant-rich diet.