Is it possible to load different "_layouts" with ajax for certain pages? I've followed this tutorial by Zack Spear. It leaves your website with a header and footer that stay constant while the body of the page is loaded in through AJAX when you change pages.

Is it possible to have different _layout templates used for different pages with this approach or can it only be one? Could I hide the header on certain URLs? Could I show a secondary menu on specific urls?
Although you couldn't necessarily change the _layout template with the method given in that tutorial. What you could do is include the header 
the templates rather than in the _layout.html for each template and change it out with another header in the other specific URLs. Or hide it, or whatever you like.