More often than not when using multiple templates on a site the templates are variations of one template that begins with cloning the primary template to a dir of another name.
EG:
1. /templates/MyTemplateName
2. /templates/MyTemplateName_forum
3. /templates/MyTemplateName_gallery

In this case there are 2 options:
1. Clone the whole template dir tree
This gives a completely independant template but all paths must be altered in the template index.php to suit the new template name.
2. Clone only index.php and the /css
This allows all includes to still point to the primary template reducing redundancy while allowing the 2 most important files to be unique.

I do the above when I want the possibility of 3 cols everywhere except the forum and/or the gallery. Since the needs of a forum are about the same as a gallery, both need the full width with 1 main column, creating /templates/MyTemplateName_special is adequate for both.

A side note: forums do not work very well when assigned to a different template and require the "Default" = MyTemplateName_special and the primary site template be set to "Assigned" = MyTemplateName. The reason it needs to be done this way is each forum post is unique and cannot be part of the Mambo menu structure so it will be loaded by the Default template. This is a work-around and will require that every new menu item be assigned to the primary template MyTemplateName as opposed to the default template MyTemplateName_special and will be necessary until Mambo allows assigning a template to a section or category rather than only to a menu item.