Yes, you can use activated sourdough starter in our recipes. My own sourdough starter, after I activate it from the fridge, is about half water and half flour. I’ve found that about 1 1/2 cups of activated sourdough starter works well in our full-batch recipes, which make 4 to 5 pounds of dough. This means that you need to decrease the water in the recipes by 3/4 cup, and the flour by 3/4 cup.
So, having done this, do you need to use commercial yeast in addition? I found that I still needed some yeast in the recipe, though I could use a lower dose, which I’ve posted about before in the context of our yeast-risen recipes. That seems like a good compromise. I did experiment with zero-yeast versions, but I found them a bit temperamental– didn’t store terribly well so we decided not to put that in our books… yet!