If I paint Harley Sportster exhaust headers with the XTC Primer and XTC paint, will it be okay to do an exhaust wrap after the paint is properly cured?

Yes, if you temperature cure the XTC Primer and XTC, you can put an exhaust wrap over it with no issues at all.

May I use XTC xtreme coating on mufflers with light rust?

Yes, you can use XTC on mufflers. Get a coat of XTC Primer down first and then 1-2 coats of XTC.