@whoisashygirl My iMac has a dual boot setup on its 1TB Fusion drive. I used Disk Utility to create a new empty volume (with no size limits) and installed a Catalina to that. It only uses around 15GB if you stick to the supplied apps and avoid filling up the Photos & Music apps.
I also have a made a clone of the old Mojave setup from my MacBook Pro (which only runs Catalina) on a 500GB ext SSD, so I can still access my 32-bit apps.