i'm confused - how doi combine both techniques?
A pre-ferment just refers generically to anytime you take a portion of the flour and let it ferment for a long time before adding it to the final dough. So sourdough starter is a type of pre-ferment.
A poolish or a biga are also types of pre-ferements, but unlike sourdough starter, they use a small amount of commercial yeast instead of the wild yeast and bacteria culture in sourdough.
You can add yeast to the final dough for a sourdough bread (it will rise faster but have less sourdough flavor), but you shouldn't add any yeast to the sourdough starter pre-ferment. And there's no need to use a commercially yeasted preferment when you're using sourdough starter.
I'm trying to find ways of making a lighter wholewheat SD bread.
Me too. It's not easy. But I've learned a couple of things:
1) Make sure your starter is super active.
2) Use 1 TBS butter per loaf and replace up to half the water with milk. The crust isn't as crispy, but the loaf is lighter.