Not many small fish are non schooling fish, see my 29g list? They are all schoolers, hence why I am moving them to the 45g lol
I have Swordtails, much like Platys. They are a wonderful LOVELY fish, I adore them with all my heart. But the one thing I would have concern with is.... Platys are too peaceful and are not fast on the swimming chain. Platys can be of nice colors, which might make them a target for Betta. They are so dopey, they would never be able to swim away from a Betta attack in time.
But they are a small fish, only 2" adult size. I am not seeing anything about them having to be in groups, I have 4 female swords and a male in my tank, they are quite happy at 5.
Tetras are a nice idea too, they can be fin nippy so be careful of types you look into. My Black Neons are sweethearts, as well as my Harlequin Rasboras. Glowlights Tetras are sketchy. My Red Eyes are fast and chase. Thankfully I have 6 and they only pick on eachother lol I am unsure of other tetras as I only learn as I go.
My Harlequins (the ones out of focus)
My Black Neons (top of picture, black/white stripe)