6'8" is an ADA Code, not an electrical code if light on wall. The minimum light height is not a requirement in respects to the ceiling, it's the candle foot power requirement and mostly in respects to visible light that shines not from the height of the ceiling that it shines from, again go meet the ADA requirement, or your local Codes respects.
I actually went to install a osprey ceiling fan last week with metal blades
and the booklet recommended a minimum height of 9 feet.Had a hard time
explaining the danger of installing on a 7'6" ceiling to the homeowner
with a small child involved.It finally went back in the box.At least untill
were out of the house.