I used the DellEFI 1.07 bootloader with a retail 10.5 disk and an 8gb HD. I did everything all the forums said about getting the install on an 8gb drive, and I always came up short. No worries I have a USB HD, I partitioned a small portion of the drive for the install, the other portion I put xslimmer and the combo 10.5.6 update. I used the I USB key method to install OS X - using the mini - to load the OS onto the external, update, slim, and every space saving technique. Here is where the problems started. I did a restore using disk utility to the mini's drive, rebooted without the USB devices. 'operating system not found' came up in the bios. This is how i fixed that.
boot with USB dellefi - will boot into os x from the mini drive after you esc, and and select drive.
run the about this mac package again - and the dellefi - but since it was already run on the USB HD you need to manually reinstall the bootloader, nothing else.
All should work now, and you don't need to reboot, but you should (without the usb bootloader) just to double check.