ATK recommends using Bob's Red Mill white rice and brown rice flours as opposed to other brands in this flour blend. Some rice flours tend to be a bit coarser than Bob's Red Mill which can negatively impact the texture of baked goods.
Be sure to use potato starch and not potato flour.
Tapioca starch is also sold as tapioca flour and they are interchangeable.
For a dairy free version of this flour blend use an equal amount of soy milk powder. You can omit the milk powder altogether, however baked goods won't brown quite as well and they will taste a bit less rich, especially in recipes without a lot of fat.