is it possible to have a default category that shows up like default.. IE index.php would have a default category of 15 but if you click on a link it would show other categories if you have like index.php?cat=16

you must create a stub file for your blog to do that.
inside the stub file, before $blog= , add the following lines:

require(dirname(__FILE__).'/b2evocore/_main.php'); 

param( 'cat', 'integer', 15 );
 

15 is the default for param 'cat' that can be overriden by the URL.
$cat will always be forced to integer.