Balancing an insurgency can be very tricky. And a lot is going to depend on the map you're using, and how many friendly AI groups you allow to spawn. Though rule number one to me is the less insurgents you let spawn, while keeping things populated enough to keep the mission interesting, the better.
What I usually do is, first things first, is build my bases/FOB's/outposts at all of the military places on the map. This accomplishes a few things, but mainly it gives you multiple places to respawn and resupply at, but also takes the military objectives away from the insurgents keeping their overall numbers lower on mission start (put blacklist markers around all your bases so the insurgents don't spawn there).
That allows me to let the insurgents spawn at civilian objectives only, which lowers their initial numbers a lot. Depending on how many civilian buildings the map has, I usually let them spawn over the entire map, and either use no filtering in their civ obj module, or only exclude them from "tiny" objectives.
On a 5x5 - 10x10 map, generally speaking, I keep their initial spawns to between 30-40(ish) groups (I use a profile counter for this which I can get you later), and then set the civilian spawns to high or extreme that way they have ample people to recruit and turn darkside.
I also usually set the CQB to 10%/pairs and place an IED module as well to keep me on my toes.
On some maps, where I can't get the balance right, I instead set the Assymetric civ obj module to spawn objectives only, and then manually place all of my insurgent spawns (making sure they are virtualized) so I have better control over the numbers. And I ALWAYS , at the end, sync the Assymetic commander to the other side's military objective module, that way they are aware of them and will attack them eventually.
Once I have the general layout done, I run a lot of tests. I generally prefer to let the player(s) do the bulk of the fighting so I tend to set the player side's military and civilian obj modules to only spawn at medium and large objectives. Anyway, the main thing I'm looking for when testing the mission is that both sides don't lose a ton of units within the first hour of play. It's not unusual for you to have 40 vs 40 groups, and within a minute or two of mission start, seeing one side drop to 30. If that happens I rebalance things and test again. I like relatively even battles. At least at the beginning.
Regarding your question about them moving out of the AO, the reason I like to let them spawn around the entire map is because I find they don't move out of smaller AO's with any urgency. This can make the first few hours boring as hell. So my preference is smaller clusters, but around the entire map so you can have fun exploring.
About the vehicles, you actually have it the other way around. The Assymetric side will never replenish their vehicles beyond what you let them start with. When they recruit civs they become foot soldiers only so be sure to start the mission allowing vehicles to spawn (or manually placing them yourself and virtualizing them). Once they're gone they're gone.
I'm also fairly certain Massi's Takistani's are broken and won't spawn vehicles regardless. Community member Davidoss made a patch to fix that. Let me see if I can dig that up for you.
If you want an example insurgency mission that you can look at in the editor, I have a few on BIS forums (and more on the way) if you don't mind downloading mods.
https://forums.bistudio.com/topic/192272-alive-spcoop-1-8-inshallah/
https://forums.bistudio.com/topic/190913-alive-spcoop-1-8-prophet/
