I'm no Mac user, but I've got an old iMac G3 a while ago. The first thing I did was to format the hd and install Ubuntu PPC. Install worked fine and the OS is running nicely. The machine hasn't got enough RAM to run the default desktop though, gotta try again with XUbuntu. All I did was download the ISO and boot the machine with the CD in the drive while holding down some key combination so it would boot from CD. Ubuntu also installed its own bootloader (which wasn't GRUB IIRC) which was text mode only but did the job. Can't say anthing about an existing install of MacOS, but I presume the bootloader would "see" the partition and create an entry for booting it in its menu.
EDIT: correction, of course the default Ubuntu desktop works in my iMac, it's just quite slow and sluggish.