If you have cam impingement then the least damaging to the hips way to walk would be toed-out, is this correct?

Maybe?: The best test to look for cam impingement is hip flex ion and internal rotation, so hip extension with external rotation with your toes out would be exactly the opposite position.

Not necessarily: With hip impingement, the best way to walk is comfortably. Physical therapy can help you get more aligned but trying to forcefully change your gait can often cause other issues at other joints.