Hi JP! The recipe calls for all-purpose flour so we recommend using that. You're welcome to experiment using bread flour if you'd like, though you'll likely need to add about 2 teaspoons of extra water for every cup of flour to account for the added absorption. Sour salt will definitely help. Sourness comes from the pH levels and bacterial makeup of each individual starter. Something that can help bring out acetic acids which are tangy in flavor is to shape the loaf and then let it rise slowly in the fridge overnight covered with plastic. There's no need to let the loaf come to room temperature completely before baking, but you want it to become puffy. 

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