This should be possible, since everything you mention are realm-specific settings (i.e. you create one realm per customer), including that a user can be admin i

This should be possible, since everything you mention are realm-specific settings (i.e. you create one realm per customer), including that a user can be admin in one realm only. I'm saying "should be" because personally, I have only used single-realm setups in production so far. Must say I'm a big fan of Keycloak.