Nobody will ever use less than 640MB of RAM with Windows Vista.
-Steve Blammer.
There, that should be more suitably updated.
I had a finely tuned and tweaked 8.1 install on a Power Mac 7100, but I messed up something (tweaking finder fixer to adjust Finder RAM allocation). It was running with 8.1 for a t least three years.
I had to install 8.6 because I needed files in that computer and the 8.1 archive wasn't with me.
8.6 is slow compared to 8.1 on a Nubus PPC (that said I have 9.1 on an 8100/110). It also uses more RAM (26MB vs. 16MB).
You could get by fairly decently on 7.6.1 or 8.1.
8.1 has the advantage of supporting HFS+ (but please use HFS on the startup partition, even though it supports it 8.1 isn't very good at booting from it), lots of modern stuff: CarbonLib (old version though 1.0.4), OpenGL 1.2.1, Java 2.2.6, InputSprockets, Quicktime (5.0.5), etc.
The Finder utilities from Jerry Kindall
http://www.jerrykindall.com/download.asp help speed up Mac OS installs.
For the 6200, I would use not more than OS 8.1 with complete patches such as OpenGL (which replaces some 68K code with newer PPC code), Game Sprockets 1.7.5, etc.
But if you can make do with 7.6.1, that would be better. You'll get faster performance.