XCode on Mac mini

The requirements would be provided in the specs for Xcode. I don't know if you can find older versions because Apple is more interested in having you upgrade OS X. Thus, they remove from downloads any older software versions. You can search their library at support.apple.com/downloads. Or become a developer to gain access to their software library.

Older versions of Xcode will not support changes made in newer versions of OS X meaning software you develop may not run on all versions of OS X.

Yes, if you can find a copy that's compatible. The current version of Xcode requires Sierra.

I have the same mini, but with Sierra. Xcode runs just fine on it.