Yes you can, only requirement of an Id is to be unique

You can use your own ID from your system as long as it’s unique like @Emre said.

Here’s the spot in the docs that mentions using your own ID https://www.revenuecat.com/docs/user-ids#provide-app-user-id-on-configuration

To answer your question “Do my users have to register with google to use RevenueCat?”: There is no requirement for your users to register with Google to use RevenueCat. RevenueCat is a service that ties to your Google or Apple developer account. The users don’t necessarily have to be aware that you are using RevenueCat