When you begin to see nitrAtes, it means that the bacteria has established enough to finish the cycle. Once the ammonia and nitrIte have become 0, the cycle is complete. You could test it if you like - put some ammonia into the tank and check to see if its gone within 24 hours or so and become nitrAtes. If so, you're ready to add fish. If you're using media from an already cycled, well established tank, the cycle should be significantly quicker. A cycle without pre-established media could take several weeks, so I'd take it from there.
I would add the rasboras first and then the betta, as it can help the betta to accept the rasboras. That way, they aren't coming into his territory - he's going into theirs.