Redux is for local state management for the application where as MongoDB Realm is the offline first database. Yes you can use redux and realm together and can create very good application. I have build a moderate complex app using the both and trust me realm is so good. I have used partition sync because at that time flexible sync was not recommended for complex app but now realm team is recommending flexible sync and I am thinking to create an app using flexible sync and I am pretty much sure it would be great.