You should be able to get that mini to work, if you boot back into target mode, connected to the other Mac. Download, and install the 10.5.8 combo updater, choosing to install that update on the mini. Here's a link to the correct combo updater: http://support.apple.com/kb/DL866
Once that's installed on the mini, you may be able to boot to Leopard.
Remember that even though you can boot to an unsupported version of OS X - you may experience other issues, such as video artifacts, or other hardware issues that might be unsolvable.
What software do you have that will not run on 10.6 ? Perhaps there's an update for that which now supports 10.6?
I've downgraded several different Macs to 10.5 - not always with good results.
jbarley - the mini won't boot to the 10.5 installer, so target mode from a Mac that will boot to 10.5 is one solution to install 10.5 - Then need to update to 10.5.8 before attempting to boot from that mini.
Last edited by DeltaMac; February 13th, 2010 at 01:11 PM.
Serendipity is a lucky guess !