I don't know about the t-shirt issue, but I'd guess that putting a cotton one on top of your tech one will have two effects:
You'll get hot - if you're used to having only one layer.
The cotton one will absorb the sweat, which won't be so likely to evaporate?
I use glucose tabs or jelly babies sometimes, but you should try out a few things and see what suits you, but BEFORE your race. I wouldn't suggest eating during your race if you've never done it before. Plus, if you've never needed it in training, you probably won't need it in the race.