The thing that will happen is over time the sand will migrate to the bottom and the gravel to the top. In a tank that small I doubt you'll be having substrate more than around 2.5 inches deep, and that's where the gas pockets start becoming a problem.
Hmm, I may not have been clear; in a tank that size you probably won't have enough substrate for pockets to form, so you could go ahead with your idea. Or, you could do all sand. Or you could go with eco or flourite since you won't have to buy very much.