I a not sure I understood your post. Are you asking how to resize a OS X partition w/o losing data? Well, if there is OS X 10.5 present then you should be able to resize drive partitions with the integrated disk utility tools. No need for Linux unless you want to pay around with Linux.
If you want to try out MorphOS w/o touching the hd at all you can boot straight off the CD and use the system. You may also do a hybrid CD/usb setup. Just use the basic boot system from CD (boot.img) and install the actual MorphOS system to an usb drive. The mini's firmware isn't able to boot off usb, but the boot.img contains usb drivers, thus once these are loaded you can proceed from an usb drive.
