From what I noticed, there is two ways to expand the warehouse/stable.
-Blueprints
-Expand Scrolls

my question is, do they work together? For example, if my warehouse can hold 30 items right now, and I expand it to 40 using Expand Warehouse Scroll, will using Blueprints: Warehouse (lvl 2) make space for 50 items (like it would without the scroll) or 60 items (50 from the blueprint and 10 from the scroll)?

They do stack, yes. So it will be scroll + blueprint + scroll etc.