every game is a newbie friendly game if you're willing to throw caution to the wind :P
Just a couple things I'd like to mention since I own both guns you mentioned.
p90 gear is a bit strange, some mag pouches (condor) can fit very very tight but I really like the eski stuff from toronto airsoft, only problem is that it only comes in black. Slings for p90s can be a bit of an issue too, and you probably don't want to pay for the screen accurate stargate ones unless you're doing a costume. Also placement on the body is weird since they're so long. I have 4 mags on my leg, and if I really need them, 2 more stuffed inside my vest. I've seen a few people tall enough to wear the mags on a plate carrier on the chest, but I can't, since they restrict my ability to bend.
for g36s, pouches for the mags can be a bit of an issue. Since the mags have those knobs on them... what you could to is link 2 mags together and go shopping for pouches that fit the 2 together without catching or stretching too much. I fixed this issue by getting the uber expensive magpul pmags for g36... Saved me from having to buy new pouches since I can use my m14 pouches with these mags. So it sort of evened out.
The rest of the gear and color choice will be up to you. :P
If you're playing a lot of outdoors, I recommend water carriers/camelbaks. Indoor mainly? not so important unless they're super long games. You can stuff a bottle of gatorade/water in a condor large radio pouch at the very worst.
Full seal military goggles and mesh lower mask combo works well, there's not many full facemasks I would trust outside of the paintball realm, so stick with paintball solutions if you want a one piece, just make sure you keep ventilation in mind foggy goggles are just as dangerous as no eye pro.... tripping and falling down a set of stairs cause you can't see sucks balls.