Of the fish you listed, I would be the most concerned about the guppies and betta not getting along, as again, it has to do with the bright colors and flashy fins. Will it work or not? Can't say, it depends on the individual betta and guppies. I've seen a tank that had guppies and two bettas living together in it once. I've also heard of plenty more stories that didn't end so well and resulted in either dead guppies or bettas.
Can't be sure about the mollies, while I doubt there will be any aagression on their side towards the betta, the betta might get a touch hissy over them. Knew someone many years ago who kept mollies with a betta in each tank, he always touted that the mollies had to be darker than the bettas, so black mollies were the best with bettas. Do I know if that was really true? No, not at all. I've never kept mollies with a betta.
The pleco won't be a bother to the betta, the only time there may ever be a problem between those two is if the betta gets pushy and tries to evict the pleco from his favorite cavee or hiding spot. Then a pleco can take a moderate sized chunk out of another fish (a decent sized pleco doesn't take crap from any fish, not sharks, not gouramis, not even oscars). But normally, these are rather laid back fish, and seem to go by the motto 'you leave me alone, I leave you alone.'
The fry are likely to be snacks for the other fish in the tank betta or otherwise. But know that bettas are carnivores, and so a baby fry looks like a perfectly tasty snack to them.