My app initially used GoogleCloudMessaging.register(String... senderIds). The app has the use-case of having multiple senderids. When this API was deprecated, we migrated to InstanceID #getToken (String authorizedEntity, String scope). We pass authorizedEntity as comma separated sender ids (e.g: senderId1, senderId2). When a new sender id (say senderId3) is to be registered, we call InstanceID#deleteInstanceID() and then call InstanceID #getToken with the new senderId appended, i.e senderId1, senderId2, senderId3. Then server apps corresponding to all senderIds were updated with the new single GCM registration token. This works fine.
Now, when FCM is used, I did the same approach with FirebaseInstanceId#getToken(senderIds, "FCM")and FirebaseInstanceId#deleteInstanceId(). This seems to work fine. But, I would like to get an official confirmation from FCM team if this is good to be used in production.