Mac OS 9.x will run on a 6100, but is tricky to install. You need a retail install CD (ie a full generic install disk, not a model specific disk) and to boot from the CD. You can't boot from Mac OS 8.6 or earlier, and then run the installer.
Is it worth running Mac OS 9.x on a 6100? Possibly if an app requires it, but most will work with 8.6. The main advantage of 9.x is the extra compatibility with OS X (eg file sharing).

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'Bout to say, yes, I knew someone who *definitely* had 9 on a 6100/60. And what a fantastic machine it was. Or... not.

I personally have a grudge against 8.6 from having to deal with a Photoshop machine that was running it and would fall over at the drop of a hat. But given that general consensus seems to be that 8.6 was one of the more stable Classic OSes it could just be that the machine-specific 8.6 that shipped on early AGP G4 towers was garbage.