1) Sugar gliders will stop eating when they are full so they pretty much tell you how much they eat, I think the average is about 2 tablespoons of fruit/veg per night along with a protein source. I always try to aim for a little bit left over in the morning, that way I know every glider has had enough, especially when you are keeping a few together. If you get clean bowls then you wouldn't know if 1 was going hungry.
2) Besides fruit and veg they should have a protein source and various suppliments.
3) They don't HAVE to have insects, they're part of their natural diet in the wild (as obviously boiled chicken and scrambled eggs are not) but we have adapted diets to still make them suitable without the need to feed insects. Some people still do, some people feed them a few times a week as an enrichment treat, some can't stand to feed them at all. It's down to you.
4) I wouldn't say there was any special food for them, they wont get everything they need just from fruits/veg and a protein source (unless the protein is from the HWP powder which is supplimented already). I have Bee Pollen, Acacia Gum, Glider Cal and Glider Booster that I suppliment my guys meals with :)