navigate (i.e ?Previous & next?) limit to a single category

I have a category (7) in my blog that is distinct from the rest of the blog. And so I’d like to have the navigation at the top of the page only navigate to posts within that category.

I’ve already set up a template page (category-7.php) for the category … however, when I click on a post, it uses single.php to displays the comments, and herein lies the problem.
From within single.php I have the navigation at the top of the page that links to the next and previous posts for the entire blog. Is there anyway to restrict this to just posts from within category 7?

Secondly is there any way to have a customize single.php for this category?