That sounds like a unique and exciting project! Stacking and connecting heavy-duty pallets to create a sturdy base for a 9x15FT umbrella is quite creative. To ensure the stability and safety of your setup, it's crucial to choose the right casters with an appropriate weight capacity.Considering the weight of the pallets and the umbrella, I'd recommend going for heavy-duty casters with a weight capacity of at least 500 lbs each. This ensures that they can easily support the load and provide a stable foundation for your umbrella.If you need any assistance with pallets pallet freight. It's a reliable service for pallet delivery.