I bought a 5" counter weight which was a waste of $1.95 so your 6" weight is just fine. I am of the opinion that 5" pads work best vibration wise. I am making a stick-on-loop pad out of the stiff 6" you get with the PC and experiment with LC 7.5" pads. (You got to learn by doing sometimes.)
I have one of those "flexible" backing plates Autopia's store sells and was not that impressed with its "flex". MAYBE CMA's yellow plate is not like that and has better flex. (BTW, I really like CMA's pads because they have more loop material than Autopia's LC 6".)
I bought the 5" Contour backing plate from Porter Cable with the heat/dust holes, not that that matters much either way, and couldn't be happier. It has much better "flex" ("compression" might be a better word) on the contact (hook) edge while keeping "stiffer" top firmness (nearest to the PC). I'll post a picture if anyone cares to see it. I'm realy surprised I never read about anyone else using the Porter Cable Contour pad.

Anyway, just one guy's opinion and preference I guess.