Howdy Don and rcruz and welcome to AGO.
I also have the HF Polisher I have both 3 1/2" & 5 1/2" backing plates. I use B&S 5 1/2" and 4" pads. Also have 5" brushes for carpet and seats. I have not done any mods... yet
Don you can use 6" pads on the 5" BP and 7" pads on the 6" BP.