My vague understanding is that the GeForce 6200 is only "supported" under OS 9 to the extent that OS 9 is able to use it as a dumb framebuffer at whatever its Open Firmware BIOS detects as your monitor's "Native Resolution". (Or 640x480 if autodetection fails.) It really is fundamentally an OS X-only card. The reason it's so common as an upgrade for older Macs is that 6200s are A: dirt cheap, B: easily flash-able, and C: it's the weakest, cheapest GPU that supports Core Image acceleration. (Also, for Cube hackers there's D: The 6200 is a low-power card and produces much less heat than an older card of similar speed.)

Not having one the only thing I might suggest is booting into OS X, setting the video mode and color depth you want, and rebooting into OS 9 to see if the values stick.