I have bettas and tetras together. 9 betta girlies and 11 neon and glowline tetras, tank is kept at 78 degrees. A few tetras didn't make it past their first weekend, but the ones that did have grown noticeably! They all hang out as one school even though they are different types. The biggest will now steal betta pellets if my bettas are slow to snag 'em. The bettas return the favor by hoovering up any micro pellets that don't immediately sink.
The Boy Feesh: Indigo CT, Lavatail CT, Ghost CT, Chicory HM, Redfish VT, Magic DT
The Girl Feesh: Christmas CT, Shine VT, Snowflake CT, Stripes VT, Midnight CT, Grif VT, Tangerine VT, Wraith CT, Amethyst VT, Ice CT, Poe CT, Turquoise II CT