Well. I kind-of feel like I wasted my time typing a rational reply...
A balloon molly will mate with a balloon molly to produce the millions of balloon mollies you see in shops - they don't come from the cabbage patch or a crane.
A balloon molly will mate with any other kind of molly provided that it physically can; ie; a male balloon molly might struggle to mate with a normal female molly purely because of it's rubbish capacity for swimming due to being deformed, compared to the healthy female. Unless you tied her down for him, but that's a whole new kettle of kinky fish. Or, if he was also a lyretail in addition to being a balloon molly, his gonopodium might be too long and bendy to be useful for anything whatsoever. These are the only obstacles I can think of, unless one or both of the individual mollies involved happened to be sterile, but that would have nothing to do with being a balloon or not.
I used to have a male guppy who tried it on with swordtails of either sex but wasn't interested in other guppies at all.
I have a male swordtail which repeatedly tries to do the bad thing with my male Boeseman's rainbowfish (which he really doesn't appreciate).
I've seen molly/guppy hybrids, platy/swordtail hybrids... honestly, if it is at all in any way remotely possible, livebearers WILL find a way.
The skill with livebearers is in stopping the wrong fish from breeding with each other. I had a lovely male saffron sailfin molly out of one drop but I got rid of him as soon as I saw his gonopodium thing start to develop because he had flared, deformed gills. As pretty as he was I didn't want to spread his genes through the rest of the females in the tank.