Not with a rasbora. Rasboras should never be by themselves. They are schooling fish and need at least 5 fish of the same species to school with. Otherwise he will try to school with your betta and treat him like another rasboras. And how do rasboras treat other rasboras? They nip!
So unless your betta is due for a hair...err... fin cut, get your rasbora 5 friends before you put the betta in his tank.