It's true. Mac OS 9.1 <IS> incomatable with Mac OS X Beta (Both the first and second builds. I dunno about this recient 3rd build). This issue <IS> resolved in the final release.
HOWEVER, it is possible to have Mac OS 9.1 and Mac OS X on the same HD. Here's how...
Two partitions. Install Mac OS 9 on one partition. Do all the updates recomended by Software Update (That is, everything EXCEPT the 9.1 update). Copy the whole shebang over to that other partition you created. Install
Mac OS X on the second partition. Then UNMOUNT your Mac OS X partition. THEN install the 9.1 Update on your first partition. Then REMOUNT the OS X partition. Works like a charm for me.
Mileage may vary. There was an odd little quirk that developed after I zapped the PRAM once. I couldn't boot into ANY partition. I culdn't een bboot a CD. OF had to be reset manually (set-defaults). Then it worked fine.