I have a single weblog with lots of categories and a single entry can be linked to multiple categories.

What I want, is for an entry linked to selected categories to use a different template group when it is selected to view the full entry.

My first thought is to add a custom field to store the template group per entry (if I add the field to an existing weblog can I populate all existing entries with a default template group?)

Any other suggestions?

The existing site structure can be viewed here.
I want individual entries in Coaching (for example) to use this template group when the full entry is shown.

I am using EE 1.6

How about using conditionals based on URL segments?

I’m not sure how I could use URL segments.

If I list the entries summaries by category how would I get a unique URL segment for the link?

The fact that some entries could be filed under multiple categories stops me using the category in the url doesn’t it?