i know a lot of it has two do with gender of the fish. males tend to be harder to keep with tank mates, either they a attack other fish, or b are attacked by other fish. tetras and guppies are usually a no no with bettas, tetras because the will nip at the tail. guppies because males confuse them with other male fish. (although i had sucessfully kept and endler guppy with one of my males without a problem.)i would assume its mainly the fancy tail guppies who are at risk of being hurt.
female bettas are a little tiny bit more laid back then there mail counterparts, they can be kept in a community tank, with no harm from other fish. i actually have three females in a tank right now with prissila and flame tetras and guppies and neither have been harmed. the females however have also been with the guppies since they were little before they colored up, so that may be part of the reason they havn't harmed the guppies.
honestly if you want to put a betta in a community i would go with a female, males need to be watched closely, and you never know if you get the crazy one in the lot. i have tried all sorts of fish with my jasper and he hates all of them but his snails. my friday however hates snails but liked his guppy companion. my other male storm has shared his tank with catfish and snails with no problem. see no one male is exactly the same.
in the end i think it comes down to trial and error with males with what you keep with them, some just don't want company, others share there home with no problems,
females maybe a little better, but then again there is no guarantee there either.
i am not sure if i helped but best of luck