Yes its possible, but not common. Most fry dont survive apparently or dont live too long.
Balloon mollies in particular have rather unhealthy lives as well, so crossing them into making hybrids is rough...
And regular mollies can cross with guppies too, but again is not common and usually results arent so good.
If you provide enough males and females of their own kind, they will usually prefer to keep to their own.
Mollies are much bigger than guppies, so first thing i would consider is that the molly could potentially harrass the female to death. And if a cross did occur,I heard about that they stick with their own species, but if I put inside my aquqrium one male balloon molly and one female guppy, will I get fry? What do you mean when you say "rough"? Do they need special conditions in order to crossbreed?