You could get platies for your tank! They don't nip at the bettas fins, and the betta will not eat them. Platy: http://www.americanaquariumproducts....tydisplay3.jpg
The Platy is a popular beginner fish, since it is relatively sturdy, easy to care for and also easy to spawn in captivity. It will not grow larger than 2.5 inches and you can keep 3-4 platys in there, since their minimum tank requirement is 5g. You can choose from many different Platy variants, including the common red Wagtail Platy. Other popular variants are Sunset Platy, Variatus Platy and Tuxedo Platy. It can survive on a diet consisting of nothing but flake food, but it will do much better if you supplement the flake food with live or frozen foods. A varied diet ensures that your Platy receives all necessary nutrients to stay strong and healthy. The colours of the Platy can also turn dull from an insufficient diet.