When you sync iCloud Drive to your mac, it stores the actual files in your ~/Library/Mobile Documents folder and that can't be changed so if your home folder is on your SSD then you won't be able to do it (and if you upload files to iCloud Drive via the web and have sync turned on, they'll end up on your SSD).
iCloud Drive is very limited in that it doesn't support selective sync and you can't change the location of where it stores the files on your computer.