I have a 100% (mostly) AP hydration starter that I'd like to take a bit and use for a 60% ciabatta recipe.
Is that possible or do I need to start anew?
Simple version: Take a small quantity of your starter(a tablespoon, or so) and begin feeding at 60%.
Mix in water and flour until you have a kneadable, non-sticky, dough. It will be close to 60% hydration.
See my starter hydration tutorial.
Converting starter hydrations: A Tutorial. Or through thick and thin and vice versa
Just in case ....