Installing Manjaro on an SSD and Switching Mac Mini Hard Drive
Rufus to use to make a bootable USB flash drive. In your case, you are suppose to boot from the flash drive to software that can install Manjaro on another drive. Usually, this is the the computers internal drive.

What you propose to do has the following flaws.


You want Rufus to put the software on the SSD drive you intend to install internal in the computer, instead of a USB flash drive. 
You have not drive to install Manjaro on to. In other words, if you did get the SSD to boot from the Manjaro installation software put there by Rufus, then this software could not install to the internal drive because that is where the Manjaro installation software resides.


Additionally, the standard RAM that came with this model Mac was 512 MB. To install Manjaro, you need at least 1 GB of RAM. If you do not have enough RAM memory, then you can upgrade. I would recommend 2 GB of RAM. One such source is OWC.

You can not install any 64 bit version of Manjaro, because the processor in your Mac mini is 32 bit. AFAIK, the latest 32 bit version of Manjaro available for download is manjaro-xfce-18.0.4-x32-stable-minimal-i686.iso.