I have successfully installed 8.6 on my 6100 with a Newertechnology G3 upgrade card. I have not seen any problems.
After running Norton Disk Doctor on my HD:
Using the Newertech upgrade card (running, extension loaded), I upgraded a minimal 8.5 folder on a zip disk to 8.6. I then started up from the zip disk (with the MaxPower G3 PDS extension _not_ installed, i.e. back to my old 60 Mhz processor). From there, I ran the 8.6 update on my hard-drive's System Folder, restarted, and "that's it that's all".