While I haven't been on a course with windsurfers yet, I do have some experience both racing and managing regattas with vastly different boats sharing the same course (cats vs. slower monos, etc.).

The first thing that I would recommend is that you arrange a meeting before next year's event with a representative of the windsurfers that intend to race. Ask what their preferences are instead of guessing. You might also consider asking them to supply a support boat, volunteers, etc. if you need the resources.

I wouldn't expect the starting line to be an issue. Start the windsurfers first, they'll be out of the way before the cats need to worry. If you start them last, it sounds like they'll be parked in the way of cats trying to start.

The other thing that you can do that is extremely helpful (especially for dinghy races with less experienced sailors) is to clearly identify a waiting area for the boats/boards between races. Put a diagram of this right in the SIs, and be sure to discuss it at the skippers meeting.

Hope this helps.

Mike