Yes you can. I've done it for years and it works great. Remeber that the PC is *really* designed to be a sander- the weight of a backing plate, foam pad, and the product that the pad is usually soaked with isn't all that different from a larger backing plate with (only) a sheet of sandpaper stuck on it.
My 7424 came with both counterweights. But I installed the 6" right away, and still have yet to use the 5". I use the 6" weight with all sizes of pads. Never had any problems with it.