Is fixed bad for your knees if you dont skid / ride high gear?

I've had knee problems on and off for years (due to badly designed legs) and all i can say is that I have found riding fixed to be far better for my knees than riding freewheel.

I'd concur with that, my experience too. If you don't start with too high a gear and build gradlally. Braking with your legs strengthens the knee stabilising muscles in a way that a freewheel never can. It is like weight training, you wouldn't go in the gym and try to benchpress 200lbs first time. Same with fixed, take it easy to begin and give your body time to adapt properly.