I'm a little confused as to the folder I should give rw access to? I didn't set aside a folder for Plex. Plex has created a folder called "Plex" and the access rights are custom.
Ignore the Plex folder. Your media can reside where you like. For example, most of my media resides in the shared folder "video" that was created by Video Station (before I moved to Plex). If you don't have that folder, you could create a new shared folder "media" and add subfolders for TV Shows, Movies, Music, etc.
In my case, I added a TV library within Plex ... /video/tv shows. Within that, I created subfolders for each individual TV series (according to Plex's recommended naming structure). I recommend that Plex has read-only access to your media files, as I see little reason for Plex to edit or delete those files.
For 2 tv series, I have Plex pre-transcode the next 3 unwatched episodes (Plex calls this "optimize" IIRC). I created a separate shared folder for this optimized content (I named it "optimized"). For this folder, I gave Plex read-write access.
The shared folder "Plex" is not to be used for anything. Plex doesn't make that real clear, but if you SSH into that folder, you will see "stay out" messages from Plex.
Let me know if I missed anything, or added to your questions.