My Mac mini came with Sierra, out of the box. I want to run Pro Tools 10 HD as I already own a full license. Is there any way to downgrade it? I really want to run Mavericks. But it won't boot to my bootable Mavericks thumb drive. Erasing it just leads to internet recovery, which leads right back to Sierra.
The rule of thumb is - no Mac can boot to any OS that is older than itself.
This rule has some slight variance at the cusp of new OS/new Mac model, but can normally be relied on.
That would mean that a 2014 Mac Mini ought to be able to run anything from 10.10 onwards, but nothing earlier, e.g. Mavericks