Can I downgrade to another OS on my Mac mini?

It depends on the version you want to downgrade to and whether or not your Mac mini can run it. So the first thing to do is check macOS compatibility for your specific model. OWC has a comprehensive guide.

Then, ideally, make a bootable macOS Installer flash drive. The reason is that in order to downgrade (revert) you almost always have to completely wipe the system drive and reinstall macOS from scratch. Download the version of macOS that you want, then create a bootable installer.

Alternatively you could install a compatible version of macOS on an external drive and boot from that (but you may still have to make a bootable installer flash drive in order to install an earlier version of macOS on the external drive).

The communities system has been flaky lately, I just noticed that the links I posted earlier don't show up. Here they are again:

• How to download and install macOS - Apple Support
• Create a bootable installer - Apple Support

Yes, if that (what?) Mac mini supports that older macOS.

No, unfortunately.