i believe you have to be rooted to use other apps in multi window mode.

 

This is correct.

You can only use apps that are listed in the multi window edit panel. You cannot add additional apps without resorting to other means.

 

when you are rooted you can delete bloatware or you can flash a rom that does not have boatware added.....so yes it can help.

 

I've found you can add apps without root but they don't all have the ability to double up like the one's on the approved list in the Multi-window list. For instance I added Boat Browser to my list but I can't have two copies running like I can with the Chrome Browser. There's an explanation how to do it in the Note 3 tips and tricks sticky at the top of the forum.