Sorry for the delayed reply, but yes, it is extremely difficult to use a single drive for both Mac OS X and Mac OS 9 backup. Primarily, it has to do with the fact that you have already started using Intel Mac operating systems such as 10.6. That has changed the bootable format for the drive to be GUID. It is possible though:
Hope this helps.
I have as many macs as you...lol
I gave up on attempting to use one drive as my master backup system since mounting OS 9 volumes under OS X 10.5 and 10.6 will eventually write info to the volume that OS 9 cannot understand and create various Btree errors when running the OS 9 disk First Aid utility; also I have not found a utility that can defrag any volumes over 190GB under OS 9; so as a general rule of thumb... Use one backup system for all Macs 8.6 thru 9.2.2 and another for OS X Macs. Also keep all OS 9 Volumes to 190GB or smaller and use Norton Speed disk to optimize (defrag).