As far as I know, any kind of molly, from balloons through to sailfins, will more than happily breed with each other. All mollies you buy in the shops are almost definately hybrids anyway; you'd have to go to a seriously specialised aquatics shop to find the original wild-type pure-bred species that actually exist in the real world. All these albinos, yellowy, white, fancy colours would have cropped up in captive bred fish from one species and then bred accross into other species. Basically, any fancy petshop molly = a whole mess of hybridization.
I love fancy mollies, but I know that my yellow sailfin mollies are not purebred examples of any specific species. The fancy colour was bred in from smaller, colourful mollies.
The balloon mollies originated as deformed mollies which were bred together to make even more deformed babies. They die younger because they are deformed, but they are mass-produced and sold to fish-shops because they are popular. Just like those dog and cat breeds that are bred to be prone to breathing difficulties, or hip dysplacia, or eyelids that fold in and scratch the surface of the eye. They are still dogs and can still breed with other, healthier kinds of dog. Just why would you?
I know I'd be severely annoyed if someone put a male balloon molly into my sailfin molly tank; just incase he spread his poor quality genes around!