I know MacMini's didn't come with 10.2 or earlier, but how stupid is that you can't install a previous OS if you want?
With the Amiga it was possible to start WB1.x on AGA-machines and PCs were DOS-compatible for a rather long time (maybe they still are), but that was done dragging along alot of old baggage both in HW and FirmWare/Kickstart.
Old real old versions of OSX expect GFX-cards at the level of an ATI-Rage and KBD/Mouse connected via ADB. On the Mini they get a Radeon and USB.
While there might be a compatibility mode for these it's doubtfull it exists for stuff like the memory-controller or IDE.
And why should Apple even consider such a scenario ?
Every Mini was delievered with a copy of Panther or Tiger supporting all it's HW, no sane user would see reason to downgrade.
Ok. it's a bit different with OS9 but here there is also a solution supplied.