Sometimes there are solutions where you don't expect them.
When I built the JH-720, I had a mix of 6mm and 1/4" potentiometers and switches, and I wanted use REAN collet knobs, which I bought from Maplin in England. The problem was that maplin offered certain knob diameters in 6mm, and other diameters in 1/4" only, and I wanted a mix of 3 different knob diameters, too. (See http://home.debitel.net/user/jhaible/jh_720.html) After some cursing and asking myself why a distributor would make such a strange choice, offereing the same brand of knobs with different diameters, but some with 6mm and some with 1/4" mounting exclusively, I found that I could swap the collet chucks between the knobs. They even had slightly different coloured metal, so it was easy to distinguish them from each other.
But well, that's just one brand of collet knobs, and not a general solution for the 6.35mm -> 6mm problem.
JH.