I have a male betta who lives with 4 guppies at the moment. He loves them and lets them swim through his fins
It depends on the betta IMO. This boy I have is really chill and has never flared, so usually the ones that are the least aggressive can be housed with other fish (but even that doesn't work out sometimes).
I wouldn't put him in with any fry seeing as he could eat them, but otherwise I would say go for it and see what happens.
Watch them for a while after the betta is introduced to see if anyone goes missing or gets bitten and watch out for signs that your betta is getting bullied. I had a guppy that attacked my betta and took a chunk out of his back. :/