I do that in a config file, the structure of the app is clear and you don't need to browse multiple files to understand the data logic. If there're very few reducers, thought, i won't bother...

In each reducers seems to be the good place for me. All my reducers are in one file, reducers.js for example, and i write them like this:

const alwaysNull = (state = null, payload) => state

I've seen people doing it as an object inside the reducers .. Since several reducers might be having effect on different parts of your state, my suggestion is to always have it in a initialState.js file in a folder like /reducers, and let them import it and pick the appropriated object property within ..

Depends on your App.

I guess you can say that in most cases the initialState comes from the server and is loaded into the App, this is the fastest and best way.

If you don't have anything to add from the server, the reducers are the best option i guess. Since you can be sure that there is nothing in the initialState which isn't handled by reducers ;)