There are a lot of fish that can get long well with a Betta if you look back a couple weeks in thread you will see some people who asked the same questions, me included.
For my Betta I have a 10 gal with 1 neon tetra (his partner died), 2 panda corys and 1 albino cory. Eventually I will put in more neons.
I heard that white cloud minnows, almost all tetras, otos, cories and ghost shirmp will do well with a betta. Like everyone said no long flowy fins on any of the other fish of he will likely attack. Well Ghost shrimp are debatable different people have had different experiances with them and bettas recently. Oh and I've heard that Loaches are good also. There are more but that would depend on the size of the tank your getting also. Anything smaller then a 10 gal I wouldn't recommend putting in more then a Betta and some plants.
Good luck!