I agree with SDSR, try shooting with your 18-135@100mm for a while and see how you feel. Personally I think the 100mm will be too long on the 7D (160mm full frame equivalent), and honestly even on full frame I think the 100mm could feel limiting if it were your only lens. I can't say it's a good choice, especially considering the versatility of the lenses you would be giving up for it.
I think you need to have some kind of general purpose zoom, or if you're set on getting a prime I would try something a bit shorter (50mm at the most). Check out the Sigma 30mm f/1.4, and if you shopped used you could probably get that, a 50mm f/1.4, and an 85mm f/1.8 for the price of the 100L (and you could do WAY more with that setup. Either that or a 17-55 f/2.8 IS, which is my favorite general purpose lens for an APS-C body. Or if you're on a budget I've heard good things about the 15-85mm.
So for a zoom, 17-55 f/2.8 IS
Sigma 30mm f/1.4
a) Sigma 50mm f/1.4
b) Canon 50mm f/1.4 + Canon 85mm f/1.8 (both can be found used for around $300)
Just a thought. The Canon 50 1.4 and 85 1.8 are excellent lenses, especially the 85mm and both would be great for portraits. Add the Sigma 30mm f/1.4 and you have a great walkaround prime. They would definitely keep you busy for a while and you would see a huge improvement in the quality of your images over your current setup. It would also help out with the low-light stuff you mentioned.
Whatever you do I would not get the 100L now if it's going to be your only lens. I think you're too focused on the versatility of the lens and the fact that it's an L lens, but looking at the big picture it's just not the right choice. Macro is fun, but it can get old pretty quickly and on an APS-C body 100mm is just too long to be your only lens. I'm pretty confident you would be happy with any combination of the lenses I mentioned. Even a $300-$400 prime is going to have comparable image quality (and in some cases better) than almost any L zoom. And I remember being set on L lenses and I could have saved myself a whole lot of trouble if I had checked out Zeiss lenses before I spent a year buying, trading, and re-selling almost every Canon prime under 200mm.