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.
How about using a theme switched like 'themekey' which will change the theme depending on the content type, taxanomy or other variables? We use that in muktware.com.
Swapnil