Hi there,
The classic headers are activated in Pro if you do not assign a header built with header builder in a page. The assignment is done in Pro > Launch > Headers screen. So if a page does not have a header assigned to it which has been built using the header builder, it will use the legacy classic header which is pulled from the CSS and PHP files of the stack you have chosen from Pro > Launch > Theme Options. Also, whatever header related options you see there will work.
If you need a customization for the classic header you either have the header options in the Pro > Launch > Theme Options or you can use Custom CSS code.
Now regarding the templating. A templating system is under development for Pro Headers (Not Classical Ones) which you will be using to add a header in a template and reuse it in different page headers. The next major release will have that functionality available.
Thank you.