[support] Blogs and RSS feeds

We have Drupal 6.19 working with a highly modified theme, its basically a data entry for calendars of events. I have blog module turned off and RSS removed (Commented out) from the page.tpl.php

I've been asked to build a youth fourm on the site with Blog, and RSS feeds.

Can a new application such as the youth forum use another theme in the existing installation and have the desired modules without it affecting the primary use of the site or would it be more desirable to use multisite and start the youth forum in a new database/installation.

I hope I've expressed this right, I'm at a bit of a loss with explanations. thanks in advance Roger