I would definitely keep the code for each document type in a separate stylesheet module (file). But unless you do the switching from an outside application, you will need to include or import all 14 files into your main module which does the run-time switching.
If you can't change the invoking application, you might be able to use extensions in your stylesheet to achieve the same effect. For example Saxon has an extension saxon:transform() that allows you to invoke one transformation dynamically from within another.
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference