I would strongly suggest leaving IIS alone and implement your requirements in WordPress:
/**
* Remove category base from "newcat" permalink.
*
* @param string $link
* @param object $term
* @return string
*/
function wpse_175424_term_link( $link, $term ) {
if ( $term->taxonomy === 'category' && $term->slug === 'newcat' )
$link = home_url( user_trailingslashit( $term->slug ) );
return $link;
}
add_filter( 'term_link', 'wpse_175424_term_link', 10, 2 );
/**
* Add our custom rewrite rule to the top of category rules.
*
* @param array $rules
* @return array
*/
function wpse_175424_category_rewrite_rules( $rules ) {
global $wp_rewrite;
$matches_1 = $wp_rewrite->preg_index( 1 );
$new_rules = array(
'newcat/feed/(feed|rdf|rss|rss2|atom)/?$' => "$wp_rewrite->index?category_name=newcat&feed=$matches_1",
'newcat/(feed|rdf|rss|rss2|atom)/?$' => "$wp_rewrite->index?category_name=newcat&feed=$matches_1",
'newcat/page/?([0-9]{1,})/?$' => "$wp_rewrite->index?category_name=newcat&paged=$matches_1",
'newcat/?$' => "$wp_rewrite->index?category_name=newcat",
);
return $new_rules + $rules;
}
add_filter( 'category_rewrite_rules', 'wpse_175424_category_rewrite_rules' );
You’ll need to flush your rewrite rules after adding the code (simply visiting the Settings > Permalinks page is sufficient).
Note that I’ve hardcoded newcat
throughout the two functions – if you needed this to be flexible & pulled by ID/option it’s easy enough.