I don't see why a solo girl site wouldn't do well. You can do well with just about any type of niche site as long as you have a clear understanding regarding the fundamental aspects of running a paysite. It also helps if your content is unique and not the same old rehashed type of content that you see all over the web. Get creative with the girl that your working with and make sure that your shooting with high quality equipment.
I specialize in working in one of the most overstaurated niches out there (amateur) and my site does great. The alexa rank for my paysite is approximately 13,000, the signups are coming in very steady and the member retention is great too.
So again, it is not so much the niche but what you are going to do (and how well you are going to do it) with the niche that you choose to work in.
Good luck to you!
