I have "Jumbo" Neon Tetras with my Betta, but I think It totally depends on the Betta's personality and space and so forth as said above. I purposefully got the Jumbo variety of the neons, cus I figured if they are a tad bigger the Betta might not see them as much of a snack, and my tetras all adore my Betta and follow him around like he's some kinda king. But I got lucky. It's all up to the Betta.
1 Doberman Mix: Emmy, 2 DSH Cats: Rowan & Jacob, 1 Bearded Dragon: Rainier (aka Mista Lizad Man), 1 Eastern Box Turtle: Greta, Two 5g tanks: Alchemy (Halfmoon) and Custard (Yellow VT)