Bettas? As in more than 1? Females?
You might already know this, so forgive me for pointing it out if you do... Male bettas must be seperated from all other bettas, they will kill each other, even brothers or sisters.
Females are not good 'community' fish, they might tolerate each other, but they don't enjoy company.
A single male betta might tolerate a school of cory cats, a couple ottos for algea, thats about it.
A few female bettas would need the same but might tolerate some smaller tetras and rasboras.
Bettas often get nipped at by tetras, danios, and other quick tropicals.
Bettas (especially males) will often kill smaller tetras, guppies, mollies, angels, gouramies, etc.