Having been around the Goth scene (and it is very different from the Punk scene) for years, I can say that they certainly do not all dress alike. I agree, most will wear black as their primary color. But check the styles of clothing. Everything from blackjeans & t-shirts to more traditional clothing hearkening back to the times of Gothic poets like Lord Byron & Mary Shelley. I've been in Goth clubs wearing a black Utilikilt and was well accepted.
I will also say that, for the most part, the Goths I know well and those met in passing have been some of the most intelligent and well-read humans I've ever met. Most are also,as others have said, very polite and well mannered. One of my favorite web sites is Gothic Charm School:
www.gothic-charm-school.com
(Formerly Gothic Miss Manners)
The website gives lessons on proper manners that would be good for most of the people I meet on a daily basis to learn. Of course there are Goth specific rules to follow as well (Friends don't let friends dress like The Crow,as they will be laughed at in clubs).

Anyway, as to the wearing of kilts in Goth clubs, by all means wear it. I recommend solid black although a Black Stewart would do well for a tartan. Just remember for Kilt wearers and Goths alike, these are outfits not costumes to the people that wear them.