How to use Susy with Zen theme
Do I have to delete the _responsive.scss file that comes with Zen and code my own?


Whether you delete specific files or not is incidental, but what you do need to do is replace everything in Zen's default Sass that relates to grids with your Susy-based system.

Because of the nature of Susy there's a good chance you won't need to change any of the markup Zen builds, but of course your mileage may vary depending on exactly how customised your theme is.