If you already have Catalina installed on your Mac Pro, Migration Assistant will create new User Accounts for each User Account it migrates: How to move your content to a new Mac - Apple Support. That is likely to result in the creation of at least one additional redundant User Account, and I presume that's not what you want to do.
Or ma I better off to erase Catalina from the destination and format with Mojave, then after migration upgrade to Catalina?
If you choose to erase the destination, just install Catalina. The installer will ask if you want to migrate data from another Mac or its Time Machine backup. That will preclude the creation of redundant User Accounts.
If you want to, you can install Mojave, migrate content, and subsequently upgrade to Catalina, but that seems unnecessarily complicated.