Can I save only videos, rather than photos and videos?

If you mean in photo stream, you can only add videos to a shared stream (they also have to be 5 min or less). Also, at the present time you can only view videos in a shared stream, you can't save them. This means that if you add your videos to a shared stream, then delete them from your device, you will not be able to redownload them; you will only be able to view them.