Hey there tankus,
After many hours of frustration, I thought I'd share what I've found.
All the units under your command can be selected individually or in groups, using the F1-F10 keys. If you have more than 10 units under your command, F11 and F12 cycle the 10 units visible. (F11=previous, F12=next)
Question 1:
Yes you are able to swap out the weapons of any unit under your command. (change their gear)
First, there must be an ammo/weapons crate near the unit you want to change gear.
Second, select a single unit using F1-F10. Then bring up the action menu of that unit by pressing 6. With the action menu open, select the action "Gear". If there is not a "Gear" selection visible, select the action "More" until it appears.
The unit will then go over to the ammo/weapons crate, and the gear menu will open up. Allowing you to change the unit's gear, as if it was your own.
Question 2:
When a unit under your command runs out of ammo, and there is an ammo/refueling truck near by, you can tell the unit that needs ammo or fuel to rearm or refuel at the truck by using the action menu.
Select the unit needing ammo/fuel, press 6, then select refuel/rearm. The unit needing ammo or fuel will run to the truck and refuel or rearm.
Note, if a unit is completely out of fuel, you will need to drive the fuel truck to the unit needing fuel. (obviously)
Question 4:
I don't think so, support options like Maddox's transport heli, are mission specific.
Question 5:
To capture a town, just order units to stand next to the town's capture point. It may take a minute or two to capture a town.