What your describing will work well if you have a one to one relationship with an actual viewable page and an actual page type. But if you use multiple page types to display content on a single viewable page, then this approach won't work. There are multiple ways of designing a site and these are just two of them, which is why Kentico is so flexible.
To answer your question, yes you can assign a default template to a page type, you can find this setting on the General tab of the page type's definition.