Just the movies since these take so much space.
Not entirely sure. Not in front of my Mac right now. But what you need to do is copy the Movies folder from within the iTunes music folder to your external. Then either a. or b. a should be easier.
a. Delete all the movies from your iTunes library that you just moved. Tell it not to delete the file (keep file). Then go to iTunes preferences and tell it not to copy files to the iTunes library when adding. Then go to the external and select all the movies. Then drag those files into the iTunes window. It will now add all the movies back into the library, but not copy them to the local drive, keeping your movies separate. You can then verify this with the get info function. Then you can delete the original movies folder off your drive. Go to preferences and recheck the box to copy files to the library so that when you add music or what not it still does. What you have to remember is that each time you add a new movie to change your prefences. Drop it in the external location. Drag to the library to add it. Recheck the box so you don't forget for music.
b. you don't want to know b.