1. I'm replacing the spring in the AKS with an m100 because the indoor field requires guns to be under 350fp. This is why I got the AKS, it also apparently has much better rof with a weaker spring so sounds good for cqb.
an m100 should give you 300-330fps. this would also make your gun more reliable. Just make sure that you get someone that could reshim the gears and install all the parts properly.
2. I need extra magazines for both guns... I only have the stock hicap with both. I'm thinking of getting the cheap BE 50 round lowcaps ($7 at airsplat each) because everything else seems too expensive (like $55 4 packs. I don't need 5 mags). I have no idea how to get the right magazines for the AKS, though, because apparently only Dboys and JG mags fit, and I don't want to get 47 mags for a 74 :(
If your a sprayer, the you'll need more. But I suggest you play a game or two. If you find yourself reloading on the field -- then buy more mags. Honestly, if its MILSIM, chances are, a Hi-Cap would be enough for a 2 day OP.
3. Do I need spare batteries? I hope not I never originally factored that into costs.
If you spray a lot, then probably you need spares. Or, you can upgrade to a higher capacity battery.
4. I want a smart charger, now that I'll have 2 AEGs I don't want to have to worry about charging. Aren't there cheap ones that just charge really fast then shut off? I don't want to overheat by leaving it on the trickle charger accidently.
the original batteries that came with your guns will die sooner than you think. I suggest saving your money and buy a quality batteries with a charger specifically for that type. I suggest that you consider Lithuim batteries. NiMah batts are bulky and heavy.
5. What are nice slings for the AKS or AUG? I heard the one that came with the AKS isn't reliable so I'm afraid to use it and have it snap or something.
If you're short on funds, you can use the sling of one of your Bags. Those are pretty reliable.
6. I don't need fancy anti-fog goggles, I have plenty of antifog wipes/sprays that work great. I just need a cheap pair of full seal goggles that will stop bbs and are comfortable.
There are so many availble. Remember, that what you buy must adhere to standards. Buy what you can afford.
7. How important is a sidearm? I have a spring pistol that shoots about 150fps with .12s and has a 4 round mag...
In OPs, I carry a sidearm. However, in the 20 years of games and OPs. I never needed to use my sidearm -- I used them just for the sake of using them.
8. Do I need fancy clothing and stuff? I can fit a huge amount of stuff in my camo pants pockets. I just worry that I'd break the magazine or speed loader or something lying on my side in the bushes or bumping into a wall. Most vests or rigs look really awkward and lumpy though. I thought maybe I could just dump everything in my slim backpack? Can I just carry a bag of bbs and the speed loader or do I need enough pockets for lots of mags?
Invest on a vest with Magazine Pockets, doesn't need to be fancy. Vest add extra protection, and the pockets add convenience. Honestly, khaki cargo pants, a black shirt, and a OD vest really looks great.