Is it possible to either:
use a different database for my backend and use firebase for the login only, if so how?
use firebase and somehow make an admin oanel to update, delete and verify users.
It sounds like you were trying to implement user management in your Android app, and just found out that the Admin SDK that allows such user management cannot be used in Android apps. This is indeed the case, and working as intended. To be able to use the Admin SDK you need to provide the administrative credentials for your project, which give the SDK full access to all data in your Firebase (and Cloud) project. This type of functionality should only be used in a trusted environment, such as your development machine, a server you control, or Cloud Functions.
The idiomatic approach to implementing administrative functionality in your application is to create your own server-side API (either on a server you control, or through Cloud Functions) where you use he Admin SDK to implement the functionality, and verify that the user calling the functionality is authorized. Then you’ll call that custom API from within your Android app.