Avoid Rendering in Final Cut Pro: Smaller Frame Size?
First off, there is no such coded as ProRes 411, so I have to assume you are using ProRes 422...which is a great start.

However it is not really worth the effort of trying to convert the clips so that you don't have to render within Final Cut, rendering in Final Cut is going to occur whether you attempt to avoid it or not.  In your case I believe it is a difference in pixel aspect ratio, if the PAR matches with a different frame size typically you do not have to render, however if they do not match even if both clips are 1920x1080 you would still need to render.