iCloud Custom Domain Email Question

My primary email of firstname @ lastname.com is currently a Google Workspace email address/domain and it is also my Apple ID (Apple Account) email address. I'm familiar with setting up the iCloud Custom Domain as I have a test domain there now, but I want to be certain that my Apple ID email address/domain can be set up as an iCloud Custom domain. Apple's guidance on this conflicting...in one place they say "an Apple ID can't be used", and in another place they say "a secondary Apple ID can't be used". Has anyone been successful moving their existing Apple ID email address domain to iCloud Custom Domain email? Thanks!