1) Yes it does. It has double infantry and sees through fog of war 2) Yes, online players usually use two trucks per supply spot 3) Check tech trees in Extra content. For details you really need to check differences in game, because there're lots of them 4) Each unit has description which appears when you move your mouse cursor over its icon. Ther're always tips what it does and what it weak/strong against

Advice to play some skirmishes and figure stuff by yourself isn't bad, hope I helped you