At first, when I read your OP, I wondered what the hell fixed specific rain gear was. However, it's true that exertion is usually higher on the fixed, thus you overheat and sweat more. I'm leaning more and more towards just forgoing the rain gear and wearing wool long johns with shorts/knickers, and wool tops (unless it's just really dumping). There just isn't a jacket made that will release perspiration fast enough to not make you wet and nasty from sweat. On a geared bike you can just gear down and take it easy....