If you're just starting out...I'd highly recommend that you get setup with your primary rifle, batteries, charger, protective gear, mags and rig to carry the mags first...then pick out the next set of stuff.
Way too often you'll see guys half-assed outfitted with multiple guns and are still not prepared to run at a game properly. These guys are a pain in the ass to the host and everyone else at the game since their batteries aren't charged, they're looking for goggle rentals, they don't have any water/etc... with them, etc....
When you get to the point where you're ready to run at a game...go to a game and see what else you need (or don't need).
If you're trying to duck/weave with your G3-SG1 in tight CQB you're going to find out really quick that you're going to want something smaller...either a AEG SMG or a GBB pistol.
If you're playing outdoors...you might not find that you need anything else at all.
AEG's break/break down...that's a fact of life. Most times...there's no lead up to it not working, they just quit. So...a lot of guys will take a second one (and third) out to big games where it's unacceptable (to them) to just hang up your hat and sit on the sidelines if your gun goes in the shitter. And Murphy's Law rules....even if your AEG shot great just before you packed it up the night before a game, sure as taxes you're going to arrive at the game and it's going to fail on you right away.
Some guys will outfit for a second "primary" that takes the same mags/batts/accessories as their go-to rifle. Others will have a completely different rifle (because different guns are fun!)...but then you end up taking 2 sets of mags/batts/etc to a game. Having 2 M4's that are setup the same is nice (you can set one up for sub 350fps indoors and the other at sub 400fps outdoors)....but having an M4 and an AK opens up the possibilities of choosing to play "the bad guy" op-for roles.
If you play with a set of guys regularly...then it's great if you all run similar guns, batteries and mags. Then if you do have some stuff go down the shitter you can help each other out.
Personally, I have:
- SystemA PTW M4
- SystemA PTW M4 CQBR
- WE WETTI/AWSS M4 CQBR
- CA G3
Not the smartest setup since they don't all take the same mags (PTW vs. G3) and none of my teammates run compatible stuff....but as a group we always take out at least two spare setups to big games so we're covered for gun breakdowns. In fact at the last milsim we had two guys working on downed guns right on the field while we were holding a fortified position.
For indoor stuff I'll take the two CQBR's and my pistol...for outdoor stuff I'll take the PTW M4 and the G3, and maybe leave the pistol behind (drawing a secondary is kind of rare at field games...but it's fun when you do).
But to start out...get fully setup with a primary rifle. Indoors/outdoors...short gun/long gun...doesn't matter. Reliable gun, enough mags, min 2 batteries, good charger, gear. Do that first, then move on to the next set.