People use all sized gravel, some people even use sand. Personally I use to larger sized rocks, because they're easier for me to clean. :] but as long as they are smooth and don't damage the betta's fins, it should be fine. Just make sure to rinse it well before adding it to the tank! You don't want to use dusty or unclean substrate.
And aquarium salt is generally used as a disease prevention/treatment. You can add a small amount (I do for mine) at each water change... but don't add it unless you are replacing water because it does not evaporate out of the water or disappear at all. Only add as much salt to compliment the amount of fresh water you are adding. :]