Well... I actually just did research on that but my research was for my rectangle tank kit with my specific filter. if you could tell me the name of your filter and the dimensions of your tank I'd be happy to calculate it for you! =] Or you could calculate it yourself at http://www.aqadvisor.com/
Just choose the shrimp and he snail that you want and add your betta to it as well and PRESTO! You got how many you can have in your tank.
They give meh advice on water change frequency though. I'd change more often. =] At least that's what people have told me. =]
When I get my tank set up, and planted I can have 5 shrimp, a betta, and a mystery snail and I'd be at 97% stocking.