Custom template for wiki page

Hi there,
I want to define a custom template for the wiki. how can I prevent it from using 'default' and redirekt it to one defined in my theme?

Goal is a full width wiki where the SubWiki links are in a column on the side instead of a sidebar.

I am also not getting how to implement the discussion feature as my theme does not include comments on pages per default. So I need to know where to past in the line
<?php comments_template(); ?>
to make the 'discussion' feature work.

Finally, will the chances affect the already defined wiki pages or just new once?

Thanks ans cheers, Alex