Yeah, I just use the same (6") counterweight with all sizes of pads. Note that the counterweights are designed around the PC spinning a plate (of either 5" or 6" size) with a single sheet of sandpaper on it; IMO moving a foam pad of any size, wet with product, is something altogether different.
Eh, I quit switching them back and forth. Seemed like the larger weight just suited me better. But hey, that's just me and I do sometimes lean on the PC kinda hard....
But yeah, if somebody has both weights they can see what they think works best. Last time I checked the PNs were:
699933 - 6" counterweight
847011 - 5" counterweight
I just ordered mine direct from PC at (888) 848-5175