I have a react-native application that's currently using mongodb, trick now is I want to integrate push notifications and firebase seems like the most ideal approach to achieve something like that, its well documented and all that stuff, will I be still following "Best Practices" if I used both?
Yes, using both depends on your use case. For push notifications, Firebase Cloud Messaging is commonly used. While Firebase offers Real time Database or Firestore for storage, if you’ve already integrated MongoDB, it’s perfectly fine to continue using it for data storage.