The fact that his gun does 410 fps straight out the box, dunno why it really bugs me as mine doesn't do just as well.
Uh, are you in the UK? If so, what muppet sold him that? If you're in the UK, then what he's got there is a Section 5 firearm, not an airsoft gun. Any site that knows what it's doing will tell him to put it back in the bag and play with something else, lock it to semi (which I doubt he can do), or go home.
I'd strongly suggest not thinking or talking about fps, because the question then always becomes "using what weight of BB?" You want to be thinking in Joules, using the weight of ammo that you're going to be using. I honestly couldn't tell you what fps any of my guns are shooting at, because I only ever look at the Joules number on my chrono (set to the weight of BB that I'm using), and any half decent site will be doing the same.
0.2g is fine for indoors, but outdoors you want the heaviest BB that your hop unit can lift, and your wallet can stand. For a full auto / burst mode gun, I find 0.28g is a decent compromise of price vs performance. Heavier is just fine, if your hop can lift it.
You're fine to use 11.1V in your gun. If the retailer says otherwise then they're looking for an excuse to void the warranty. But if that's what they want to do, they'll do it whatever you use, or say you used. If you experience overspin and double-shooting on semi (you pull the trigger once, the gun shoots twice) then you can use the active-brake or pre-cock settings on your ASTER to fix that.
You've bought a perfectly decent gun, and it'll do you proud if you feed and treat it well. As above, clean the barrel (YouTube has plenty of guides), give it 0.25g or heavier, and get the hop dialled in (again, YouTube will help, or ask here). Your ASTER is great, it's worth the time figuring out its features.
But
none of that matters, as long as you're having fun in game. Plenty of us have a blast using the cheapest CYMAs, shotguns, revolvers and such. Airsoft isn't about having the bestest top-tier laser-beast gun, it's about the experience of taking and making hits with whatever you have to hand. Your gun will be as good as anything else out there, once you figure it out - you haven't wasted your money.