I use the 6" weight with a 5" backing plate and 6.5" pads, I have used the same combination with the 5" weight. IMO the 6" weight with the 5" backing plate and 6.5" pad is smoother(less vibnration at high speeds).
I have also used the 6" weight with the 6" backing plate and a 9" pad. I have only done this once and so far I liked it when working on vehicles that are larger. I do not know if this is a good combination for the machine, but it worked very well for what I was using it on.