If I have a Catgirl Goth Rave in the San Francisco bay area in, oh, mid-December, would people show up?

I would! I'll be on winter break starting 12/10, and a Catgirl Goth Rave is an excellent excuse to visit the bay area. :)

I've never been to an anime convention before. Are there any social niceties I should pick up beforehand?

IME, the social niceties of anime cons are much the same as they are at any other con. Make sure to set some time aside to people-watch, and feel free to ask the cosplayers for their picture; they're there to show off!