full stock m16s mean you can go to the hobby store and find the biggest 7.4 or 11.1 you can fit into the stock. more mah = more better.
for the e&l if you're limited to stick batteries the concern is with future upgrading because they don't have a lot of battery space. Their stocks are solid. If you eventually put the newer market motors on there that run neo magnets, their current draw is about half as much more than most stock motors run, it can get a bit dangerous on the low end airsoft lipos.
Higher C and mah rating will be a generally safer battery as they can provide more amps than the motor can draw. If the motor outdraws what the battery can output, then you get breakdown internally of the cells and swelling and potentially fire.
A reasonable way to estimate the output of the battery is take the C rating and multiply it by the amp/hour capacity. So if it's a 7.4 20C 1400mah, you take 20x1.4 and get 28amps... roughly. It's not perfect but it's usually pretty good. So the estimate is that this pack can output 28 amps constant. Some batteries list the burst rating.
Burst is important as it means that the pack can withstand spikes in the draw, why that's important is that airsoft guns require a lot more power to get them started to overcome the mechanical resistance in the gearbox from a dead stop. Stock builds are fine they don't spike too much... neo magnet motor builds on stiff springs and very fast gearsets have been measured spiking into the 90s and 100s.
None of that is super relevant to your exact question though. You won't be limited in battery pack size for your m16. The EL AKs, you will be.
From my experience and direct observations of many builds that have gone across my work bench, most stock guns will draw around 14-18 amps when fully assembled. The same gun on a neo magnet motor will draw 22-26 amps as constant draw. Spiking is anyone's guess, I don't have the precise tools to measure that. Had a mutant ACM gun draw 8 amps last week. That's just nutty. Slow ass motor on torque gears on bearings. lol.
Here's a size comparison of a couple hobbyking 7.4s... the one on the top is their standard 2 panel 7.4 airsoft battery. The bottom one... well... let's just say I went into their non airsoft listings to find that one. That one goes into the full stock of my mp5.