Thought I'd weigh in with my decisions/experience moving forward, hoping it might provide helpful info:
I decided to upgrade my 2013 iMac running Mojave to the latest 2020 iMac to keep up with the current world.
-- Prior to ordering I spoke with Apple, and was told that I'd be able to start up the new "Catalina" iMac from an external hard drive containing Mojave to run legacy software. Given that I was talking to Apple sales, I treated this with a grain of salt per some obscure references when googling on this question (figuring I could keep my old iMac and or use Parallels for PCadd if necessary).
-- On receipt of the new machine (which did have Catalina installed) I set it up (security wise) to allow booting from an external drive. As I suspected, It did not work with a Mojave external drive. It did boot from a Catalina external drive, and a Big Sur Beta external drive . . . you can see where this is going . . .
-- While not too surprised, I wanted to "due diligence" so, I spent 3-4 phone calls with Apple tech with the simple question: is it possible to boot the new 2020/Catalina iMacs from a Mojave external drive. Every person at Apple tech support (escalating to more senior tech people) told me this was possible-- referring me to various instructions from Apple. Eventually the final senior tech put in an inquiry to Apple engineering. The response from engineering was. "It's not possible to boot from an external HD containing an OS earlier than the OS the machine came with.
-- For now I'm keeping my 2013 iMac as insurance just to run PCadd.
-- I've got PCadd within Parallels installed on the new iMac. I put 64GB of memory (from OWC) into this machine with Parallels in mind. Here's my report on that: It seems to work fine, with a couple of qualifications. I tried it before the memory upgrade, at the stock 8GB and observed a substantial/reassuring difference with the 64GB. Parallels is noticeably snappier and so is PCadd. The only glitch I've seen is on opening up PCadd I receive a "The defaults file could not be created" pop up message. When I click "OK" PCadd opens w/o a specific file opened, but I can use File/New or Open or Open Recent to get what I want. My files are in iCloud which works-- If I shut down PCadd in Parallels and open it on my old iMac i see modifications to files done w/in Parallels.
-- Furthermore, while it's true you can't use 2 monitors to spread the menus in PCadd/Parallels, you can still use 2 screens with Parallels on one, and reference material on the other-- copying text and importing graphics back and forth etc. This is how I've always used my 2nd monitor anyway as the iMac screen has enough elbow room for PCadd for me.
So, there it is--still hoping for getting my old iMac out of the house next summer Todd
-- but good to go for now.