Uh Oh. You are using absolutely useless method. Posts, categories(and everything in WP) can be freely accessed via different kind of url combinations,feeds,api and etc (so forget your code/htaccess method).
Instead, use php codes in your active theme’s functions.php:
if (is_category() )
{
global $cat;
if ($cat->term_id = 123)
{
if (!current_user_can('install_plugins'))
{
}
}
}
replace category id with desired one.