Michael Black is correct that it should be no problem installing Linux onto a Mac Mini. If this is your first attempt installing Linux, then you want to be careful installing it to a system you want to keep intact. Just make sure when asked about the bootloader (if a choice is given), then select your external drive and not the internal drive containing OSX. This way your external drive can be easily moved to another UEFI booting system. I've installed Debian on multiple Intel Macs side by side with OSX on an internal drive, as the only OS on the internal drive, and to an external drive which I move from system to system.
The only time I ever had an issue was years ago on one or two 2006 era Intel Macs when I was trying to install a 64bit OS because one or two models only had a 32bit EFI to go with the 64bit CPU. I don't think this is an issue these days as I believe the distros have resolved the issue, but that would be the only difficulty that may exist if your Mac Mini falls into this category.
I don't have access to a 2018 Mini, but unless the hardware requires a driver which isn't in the default kernel of the OS you are installing I don't see a problem after disabling the T2 security as mentioned by Michael Black. If a newer driver is needed, then sooner or later the open source community will provide support as evidenced by the keyboard & trackpad support for the MBPro 2016-2017 laptops being implemented in the next Linux kernel.