I run in light to midweight wool socks year round. It never got over 29°C here this summer, though, and that only for a short time - I know Singapore is quite a bit warmer!
Can't help you on sizing. I will say that wearing your socks too tight greatly increases your chances of blisters.
Edit: and wearing your socks (and shoes) too tight pretty much guarantees your toes and feet will be cold. If cold is the main issue, and you have to choose one thing, you'll be better off getting shoes and socks that give your toes and midfoot complete unrestricted movement. If you've never tried this before, you'll be surprised at how much warmer your feet stay, than they do in insulated footwear that cut off your circulation.
Last edited by OakLeaf; 10-19-2009 at 05:49 AM.
Speed comes from what you put behind you. - Judi Ketteler