ok, figured it out, instead of is_category i needed in_category for my needs
This worked for me:
<?php if( in_category( array(‘275’, ‘22592’, ‘12869’, ‘23648’,’309′, ‘6658’, ‘310’) ) ) :?>
<p>Disclaimer:text here.</p>
<?php endif;?>
Now is there a way I could put some sort of outline box around this?