Great!

Be sure to get a good understanding of skins and how to best organize before doing lots of work. You probably don't want to have "enabled" and "disabled" placeholders. A placeholder without a skin will show nothing, as you've found. A skin does not need to have an attachment for every placeholder. You can just leave the placeholder empty in skins you don't want something shown there. Even if a skin has a placeholder, you can hide a placeholder just like any other attachment by using the paperclip icons.