Any of the knobs should work just fine, I would say go larger to be on the safe side, as the Goldpoints don't require the offset hole that the stock switches do.
You can wire the switch in as is drawn out in the Foreplay III schematic, or you can wire them in as a standard attenuator (the pads on the attenuator are labeled for this).
I find that with the additional steps of the Goldpoints, I can get away with just one pair of padding resistors for all three inputs. Therefore, when I build a Foreplay III with Goldpoints, I will wire each channel from the RCA jacks right into the input selector switch, then place the padding resistors between the selector switch and the input of the attenuators.