You could get 5 harlequin rasbora's. They are schooling fish and fun to watch when they eat (dart really fast to top, snatch food, and back down) Not too colorful (IE: not too tempting for the betta) and are fast, so betta will have a hard time trying to catch one in a 10 gallon.
Here is a picture of them:
https://www.google.com/search?hl=en....,cf.osb&fp=383fd8da031cc313&biw=1600&bih=775
Here is a picture of them:
https://www.google.com/search?hl=en....,cf.osb&fp=383fd8da031cc313&biw=1600&bih=775
