For the server I am playing on, the AI do not get in static AA for some reason

RHS AA trucks are very effective if you train your AI.

Last time I checked you can't add vehicles to garrisons and everytime you save you must rebuy vehicles. Very impractical for deffense.

I agree, we need aa launcher troops!

I dont know about the effectivity yet but you can buy AA-statics and make militiamen use them.
First buy the static and place it, then recruit militiaman and add it to garrison... once you restart server the militiaman will be manning the launcher.
Not sure how good they are at using them but my pals have an airport and we have 1x mortar, 1x AA and 2x MG's manned in there. (about 50 men otherwise. :>)

Naturally if enemy attacks our places we go and defend aswell, it takes them a while since the closest airport is beyond the peninsula.

Next to useless, try and use one and you'll see they has a limited arc.

Posted on Gravyleg's later thread but I found that saturating the outpost with alot of static HMGs manned by rifleman is quite effective. I managed to unlock it by stealing enough of them. In smaller outpost i station 10-12, at larger outpost they can number up to 25-30. Make sure they are well distributed. They have a wider arc and the interlocking fire from so many guns can be quite effective. Captured a few attack helos in the process as well. You will take 20-50% casulty per large scale attack depending on the layout of the base and whether the enemy firebomb with fighter beforehand. This is with RHS USAF/ARFR mod on.