1. I generally use units in 3, 6, 9 or 12 stacks. 3 and 6 stacks tend only to be for stuff like Marines though. For your standard army stacks I always go for 9 divisions.
2. Here I usually go for 3 Arm and 6 Mec/Mot per stack.
6. Brigades don't make good economic sense unless you are nearly...