I've been told 100% WCs for most fishes isn't recommended but bettas are an exception. Every week I empty 12 tanks, fill them with new water at the same temp, add dechlorinator and plop the bettas back in. None have ever looked ill after a 100% WC.. actually, most of the boys blow bubble nests right after a WC. Of course, 50% WCs would be fine.. I just personally find it easier to do 100% changes less frequently.

The gravel thing though.. it really doesn't matter whether there's beneficial bacteria or not. You aren't supposed to wash the gravel if you want to cycle the tank. But again, a 2 gallon just isn't big enough to hold a steady cycle.. so it doesn't really matter.