1) Most likely I will use conventional organization, but with changed names. My story is a quasi-medieval fantasy.
2) The enemy is big on ambushes. So most likely infantry. But my character is a hotshot with the bow, so probably archers somewhere...it's the first stages of a war, so only small groups are at the front now, perhaps no calvary...yet.
you could explain that for every platoon of infantry there is an archer or two. They would have to be multifunction (ie, more than just archers as they would be too vulnerable otherwise)
3) My MC is a scout, and he comes across a group much larger than his unit can handle. For a reason that I haven't figured out yet, though, they *have* to take on the enemy. I've got a spiffy way for them to do so, but that can't be the *only* fight. The MC has to be in some actions before this, so the commanding officer can trust him to do the spiffy thing...
So you could have your MC doing some scouting and discover a patrol that had been wiped out by an ambush and had to fight a pitched battle to report back (there by earning the trust of his commander) You might have him acting as a scout for patrol that is ambushed, the Lt (patrol leader) is killed and the MC takes over getting the patrol out, reports back, etc. Either of these would be a smaller unit battle (squad or platoon size ie, 16-40 people)
I hope this helps some. Remember when you are doing small unit or platoon battles, if you are in 1st person, then your MC may be too busy to see much of what is going on, but he will occassionally take stock of the over all situation he is in (especially if he is leading people). I tend to write the tight single combat, mix in occasional info about the immediate area, and even more rare info on the battle at large. (if the MC is in the thick of things. if not, then having him observe larger action and updates to commanding officers is a good way of passing on info.)
I also should note that I tend to write in the 1st person.
-paul