Ythrians are unclothed because feathered. Some human choth members, emulating Ythrians, wear only body paint.
Dominic Flandry dresses as colorfully and flamboyantly as possible but not in anything unexpected like a kilt or toga.
Robert Heinlein, aware that social norms differ historically, has shorts on men in Double Star and kilts in parts of the Future History.
In SM Stirling's The Protector's War (New York, 2006), Chapter Eighteen, a hostage wears:
"...a plain gray guest-weave rather than the Mackenzie tartan..." (pp. 514-515)
I was told that a book describing clan tartans has been taken as normative. See here. Apparently, one of the Kings had a tartan designed for him but anyone can wear it. Scottish people of Italian descent have a tartan. See here.