Will my 2014 Mac Mini boot an SSD that already has Linux on it?
There is no way to know in advance if you can boot the operating system on this SDD. The the only way to know for sure would be to try booting. The Mac Startup Manger is employed to either manually or automatically determine what code needs to be loaded in to RAM memory and executed in order to boot the computer. This Startup Manager does not recognize all the possible valid boot configurations and therefore may ignore the Grub and/or Ubuntu startup files stored on the SSD. One way to avoid this problem would be to first boot the a rEFInd USB flash drive image file. A link to download this file can be found at the Getting rEFInd from Sourceforge website. If rEFInd is necessary to boot Ubuntu, then you can either modify the name and location of the startup files on the SSD to avoid using rEFInd or install rEFInd on to the SSD.