Just purchased Shutter and have two questions: (1) can I use my own videos or must they be YouTube or Vimeo? and (2) If I only have six photos for a grid, will the extra placeholders show on the published page?
Shutter supports YouTube and Vimeo embeds, not self hosted videos. You can upload your own videos to either of those services.
You only place as many images into Shutter as you’ll be using. Delete any that you don’t use.