Speaker height vs ceiling height

I do not really qualify to address the issue, but intuitively I would certainly leave couple of feet between speakers and ceiling for dynamic speakers. Panels might tolerate less space above them, I guess. And horns...I have no idea.

My basement ceiling is about the same height as yours.
My speakers are 5' with no problems.
Don't think I would try anything taller in a speaker, but I could be wrong.
BTW - Which 6' speakers were you thinking of?

I really do not believe tall speakers are an issue. I have owned 8' tall electrostatic speakers and 6' tall dynamic speakers without any problems. Most of the really tall dynamic speakers place their drivers in a D'Appolito configuration, so the midrange and tweeters are not close to the ceiling anyway. Having the speaker near room bounderies (floor and ceiling) supports bass response and I seldom hear anyone complain about too much bass.

Yes, there can be too much bass, easily. In smaller room.

The size of the cabinet is not the issue; it is the position of the driver within the cabinet relative to the boundary that causes a concern.