Hi,
If you don't set a warhouse in a shipping method, it should be available for all the warehouses.
Also, if you want to provide free shipping over a certain amount, why not create a coupon for that ? You can activate its "auto load in cart if possible" setting and use its "shipping percentage" setting to provide a 100% discount on shipping.
Also, note that most of the time, merchants will use warehouses while they actually shouldn't for what they are trying to achieve. In most cases, it's possible to do better shipping cost calculations without warehouses. But to tell you if there is a solution to what you want, I would first need you to explain what shipping costs you need to charge (maybe with some examples ?)