You could write the code in your rules page to show different rules based on a forumid variable being passed. Or, if it's a template do a condition based on the forumid and show something different based on that condition.
For the message length, you might be able to write a plugin that changes that length based on the forumid (fetch_foruminfo location?):
PHP Code:
if ($forumid == 'x')
{
$vbulletin->options['postmaxchars'] = '5000';
}
(Not tested. I have no idea if that will work.)