It did not go smoothly...
I bought the 2.53 Mac Mini, with Snow Leopard preloaded. Small problem, my Leopard disk is 10.5.0, and after repeated tries, would not boot the Mac Mini. So I tried booting the Mini in Target Mode from my Intel MacPro, and install Leopard from there. No joy, the installer program on the disk restarts the MacPro and the MacPro won't boot from the Leopard install disk.
Both of these machines were built AFTER the Leopard disk came out, so I was stuck. I even tried using my G5 MacPro, which could boot from the Leopard disk, but would refuse to install Leopard onto the Mini because the file formatting on the Mini was set as GUID (for Intel processors).
Finally, about midnight last night, I asked myself why must I restart my source computer when installing software via Target Mode to a secondary computer. A quick search later, and I found out that you can use Terminal to locate the OSInstall.mpkg file on the installer DVD. Running this file directly negates the need to restart the host computer. Finally! I was installing Leopard 10.5.0 onto the Late 2009 Mac Mini. An hour later, I was finally able to start the 10.5.8 updater which I downloaded late yesterday afternoon.
Small problem...the validation of the install package failed and was asked to contact Apple for support. Royally pissed at this point, and exhausted, I went to the Apple site and downloaded 10.5.5 and 10.5.7, and 10.5.8 for good measure. Set to download, I went to bed.
Got up this morning, and tried 10.5.8 first. It installed successfully via Target Disk mode.
It installed successfully!
Now I rebooted and started the User Migration program from my old G5 MacPro to the new Mini. The only glitch so far...the Mini restarted in Terminal for the very first time after the install/upgrade process. Not sure what will happen tonight after I examine the User folder, and try a couple restarts.
Still need to install Windows XP via bootcamp and all my programs...
Also, for anyone attempting this feat, please make life easier on yourself and buy a Firewire 800 to Firewire 400 cable. The Mini only has a FW800 port, and my legacy hardware has a mixture of FW800 and FW400 ports. This one cable made my installation attempts somewhat easier....