It was mainly designed to work with the 7" Meguiar's Softbuff 2.0 pads. The W68DA backing plate will fit exactly into the recessed part of the said foam pads. You can use this with a 6.5" foam pad but the edge of the backing plate will close to the edge of the pad, if not flush. (I've tried this with the 6.5" Meguiar's original Softbuff pads) That makes buffing out edges or contours extremely dangerous as the BP's edge can damage the paint. The W68DA has a rubber edge but I still wouldn't risk it. Will it work, yes. Is it safe, no.
If I were you, I'd use a 6" Lake Country backing plate for your 6.5" foam pads.