I've had a long-finned male betta live peacefully with a mystery snail (the beginning was a bit rough but he soon realized the antennae are not food). Long finned would work well with more kinds of fish/shrimp since they are slower - even if they want to hunt something down they can't and they give up quickly.
My female wild betta on the other hand, has short fins and she's super fast. Her mates are 4-5 snowball shrimp, 2 amano shrimp, 1 red nose shrimp and 1 oto. For some reason she'd leave the shrimp alone for the most part (she might show interest in a meal but she just swims away every time), but with the oto - she's a bully. She might chase him down or flare on him (I never knew females can flare before I saw her do that towards him) but she doesn't bite him, she just wants him out of her sight. The oto looks healthy so I don't think he cares too much - I might be more worried than he is.
So what's the point of that story? I think it depends on the betta's personality, and on its speed. A chill betta can do well with species that it might consider as food. Slower bettas (longer finned) will work better with peaceful species, but they might be out-competed for food so you have to keep an eye on that.