Unless I'm misreading you, I'd say that MOST people use the "regular" PC hook and loop (velcro) backing plates as opposed to the flexible (all-yellow rubber) ones. There's about a 1 oz. difference between the 6" "regular" and the 5" flex.
I've sometimes used the "regular" 5" backing plate with 6" pads (works fine) and only recently started using the 5" flex. The 6" plate DOES vibrate more, but it IS one of the plates the PC is designed to work with (assuming you're using the correct counterweight).
I myself always use the 6" counterweight, even with the 5" pads. It probably compromises the random orbital nature of the PC a bit, but not to any appreciable extent (IMO). And it cuts down on the felt vibrations at higher speeds.
