So, by Single shot, I'll assume that you mean One Shot and not single shooting.
There has also been some talk that shooting over 10fps can diminish AF capabilities in low light and Canon also addresses these limitations in the Manual, so you may want to limit your high speed continuous shooting to 10fps or less, especially in low light, low contrast situations in AI Servo. One Shot AF Mode performs better under these circumstances.
As far as AF point selection, if you have some slow moving subjects then I would use single point with 4 point expansion, so that you have better AF accuracy. Zone Expanded Area AF picks the points for you, so it may not pick the point that you want, especially in low light, low contrast situations. I'm defintely recommending that you place the AF point where you want it on your target, so that you may select a higher contrast area or an edge of the person or their clothing. If you leave it up to the Zone Area AF, that wont happen, since Zone Area AF likes to look for the closest part of your subject to focus on. Face detection is also poor in low light, low contrast situations, or with faces that are smaller and/or farther away.
Single point AF (4 point) area expansion may assist in low light low contrast, not because it lets more light in, but because the AF expansion area has the potential to expand to cover a larger area, so it will have a greater chance of locking AF on an area of higher contrast due to it's greater coverage area. The opposite is true of Spot AF, it will struggle due to the lower surface area that it covers on your subject.
I hope this helps!