The answers to your question are, I suspect, going to be a bit misleading given the vagueness of the questions.
In my player's immediate future, for example, I will be using/involving all eight of the weapon options you've mentioned in your first question, and troops in brigade, corps and army strength (all three firmly in your 'above battalion' option. And I will be using standard role-playing rules for it, not wargames.
Of course, most of what is going on wlll be a background of blood, guts and bullets. The PCs won't be supreme commanders so their actual physical involvement will be more on the squad to company level. Certain weapons delivery systems (MBTs, support weapons, orbital artillery, etc) will be very relevant to them, not necessarily through dodging or firing it (although both could happen) but it will become part of the environment and quite likely even a mission objective...