I love them on cold nights, or really hot days. If I can remember, Your Grace, I'll find a couple sources and forward. America Online, Inc. On occaision, I leave mine off and only wear the underlying Moroccan knit skullcap. Most of the time, the regular way I wind it and the Moroccan cap keeps my head warm enough at night, and allows evaporation cooling during the day.
Turbans can be very versatile head coverings.
Yes, ghoutras are period. What is undeniably period is a rectangle of solid cloth folded into a triangle and worn as a ghoutra, preferably with a tiraz band attached along the hem. Wearing a ghoutra or a turban over a conical helm is easy. For a ghoutra, just fold it like usual, and secure it with the igaal, the double rope thing one wears with a ghoutra. You could also wrap a turban around the helm, using the helm as a base.