Can I use a 6in pad on a 5.5in backing plate?

Your pad should always be a little bigger than the BP. So for example for 5.5 inch pads, we use 5 inch backplates. This prevents the BP from hitting anything on the side since the pad acts as a buffer (in both meanings of the term).

What you never want to do it use a smaller pad on a larger backplate, this is just begging for trouble.