Im having wonderful Success with Harlequin roseboras and Glass Catfish. Both can be found in the wild with wild Bettas thats why I tried them. I have a Betta tank on the larger side so I wanted to stock it with other things. Also If you have room I read Khuli Loaches and YoYo loaches can do well. I have a pair of YoYos and they havent bothered any one.
For clean up crew Coreys are great, and I read Otocinclus Catfish or Otos can be great to as long as you dont have any one in your tank with yummy slime coats like Discus.