For a betta to be kept with other fish, they MUST have at least a 10 gallon tank. This allows the other fish to have lots of room to swim, and that the tank can handle the bio-load of the fish. In tanks under 10 gallons bettas may be kept with small invertebrates.
I thought I should clear this up, as there are countless posts with people asking whether it is OK to keep *insert fish here* with a betta in their 2.5 gallon tank.
I hope I don't make the large red letters a habit, it is so tempting though!! ._.