Yes, you can use the SJW by itself. Either one can be used alone. My sense is that the scullcap will work more with the itchies, but that may not be the case. Starting one at a time isn't bad either, then you can see the direct effect.
I tried all the grains with the dogs here - mine never tolerated any of them, and were always quite lean because of the meat and veggie diet. Only recently my boy has weight on him though.