[Robert Withers]"I've heard that if you have Catalina on a computer you can't re-install Mojave."
I believe what you cannot do is to "non-destructively" go backward from Catalina to Mojave, but if you boot your Mac with the Mojave USB stick and you format the internal hard drive, it has no idea what operating system was on there. At that point I don't see why you couldn't install Mojave on it.
Generally in my experience and, I've been installing macs for a school since 1997, you can install the earliest OS that in in use at the time of machine manufacture. Except in the case of new models being made for the upcoming OS. The most recent example was my 2017 5K iMac. The new disk format has made it more difficult.