I prefer to use a light prop load for break-in as apparently your manufacturer has. This regardless of the way you choose to beak-in whether it be on a stand, the airplane or perhaps a combination of the two. While no you may not "hurt" anything using the heavier prop load you just may find yourself with a better engine by using the recommended break-in prop.

You never said what airplane but now is the time to start building your "propwall" Every flyer of fixed wing aircraft needs a prop selection and besides whatever that ship is you may just find out the 12x5 could work better, only you will be able to ultimately answer that by actual trial.

John