David, we have been working with the categories in the last months a lot, but not exactly in the same way.
Probably if simplicity matters (always matters) you can just add a new row to your category table that references to survey_id.
In that way you won't need to alter the survey table, no need to add more fields.
If you want to create something more nice, maybe, a single category could have more than one possible surveys? you can create a new table with three main fields,
cat_suv_id is a sequence
category_id
survey_id
The things should be different if you want convert the survey module ala general_comments, in order to apply it to whatever module you want.
Hope this helps!
