For the servo screw, something like this:
http://www3.towerhobbies.com/cgi-bin...?&I=LXE017&P=7
I think the easiest way to figure out if the center diff is straight is to spin it, and whichever combination of washers allow it to spin the longest will be the ones to go with. Otherwise, you might just need to eyeball it.
2. Just play around with shims vs. no shims, to try to get the best combination. Basically, you want the gears to be as tightly packed as possibly, but without any binding.
3. Geared properly, it won't overheat (assuming it's not defective).
4. Most people don't run more than 6s in buggies, because 4s is plenty. But just to give you an idea, my 4s 4800mah weighs about 500grams. I guess 600grams isn't that bad.