Install Mojave on an external HD or Volume within the internal HD?
So I have the brand new 16" MBP. I am running into an issue though because one of the programs that I brought forwards in my Time Machine install I am using for school doesn't run in Catalina. I can't switch to a different program because of how important it is to my class. So my question is what might be better (if it can be done)? Am I able to create a separate volume within the computer and format and install Mojave on it? Or do I have to take a separate HD and install Mojave on it and install the program into it and use the option key each time when I plug it in to boot into it in order to run that single program?
If there are other options possible then I am up for those suggestions also.
Thanks!
MacBook Pro with Touch Bar