Paolo,
I understand you have an 0.72 M6 - this one should be ok for the Nokton, but a part of the VF will be obstructed - not nearly as much as in case of the R4. Anyway, the Nokton 35 is a great lens, so a compromise is well wotrh the while. The R4 however, has problems with lenses that are longer than 2-3 cm, so it excels with the CV pancakes. I use it mainly with the 28mm, but also with the 21 and 25 Biogons. My suggestion would be, to trade your Leica 21mm for the R4A with the C Biogon 21/4.5 if you don't mind the slower max aperture.