When creating a new page a user is made to choose a "page type" which actually means choose a fieldset. So if I want to set non-editable variables for every page using a specific fieldset how do I do that?

Use Case:

If I have a "Gallery" page type/fieldset I don't want the user to be able to pick the template for a blog post. I would like to set the gallery template for all pages made with the "Gallery" page type/fieldset. I would also like to be able to set the layout.