1. I prefer the 20mm. If I am using the 20mm, I'll fire the machine-guns at the same time, it is enough hitting power to kill a plane. The 30mm is very very hard to hit with. If you can hit with it reliably, it might be a better choice for you.
2. You generally won't get slow enough in the MA to use flaps. If you are at a speed where you can use them for turning, it is usually OK to do so. As far as using them while trying to out-climb an enemy, it is generally a bad idea. You will climb more steeply without them, and the 109 is fairly stable down to 100 mph or so.
3. I tend to use my rudder to help roll the plane, to keep the nose in the right general area when I'm in a turn, and for making shots. The 109 has a fairly effective rudder, you can give it hard rudder one way to kick the nose around for a snapshot, as long as you are prepared to pull it out of a stall right away. I generally don't make 'coordinated turns' in any planes.
4. Without gondolas on offense you can be aggressive against most every plane. A Spit IX will give you trouble, but that is just about it. No other plane has the vertical capability to keep up with a 109. In a straight 'turnfight' the G10 won't do all that well, but I'd recommend using the vertical to stay above and behind your enemy as he turns, and doing one of two thing. First, you can make passes at him. These will be relatively high angle deflection shots, on the order of 60-90 degrees usually, depending on what plane you are flying against. Once you conclude a pass, you loop back up and see what he does next, and get ready to make another pass. Second, or what I normally do, is make a pass like normal, but when you are looping up, chop throttle and come around hard. Usually you will catch them off guard and unprepared. From this position you want to gradually (and I stress gradually) close to as close as you can to kill them (which in a 109 is generally 200-300 yards). If they do a break turn, you can either chop throttle and follow them through it, or do a high yo-yo and drop back down behind them once they finish turning.
If a second plane joins in the fight, it is important to see them as soon as possible. This is because you will want to start going towards the new enemy as soon as you can. If he is co-alt with you, do a slight dive and hit WEP, get going as fast as you can. Off the merge with the second guy you will generally want to do a nice lazy Immelman type merge, just to see how much energy he has- if he has a fair amount and has immelman'd and is coming back at you you can either try to rope him, or flatten out your Immelman and start a spiral climb. The only planes that can tag a 109G10 in a spiral climb are another 109G10 or a Spit IX. Nothing else can do it. Then you'll be back to where you were with the first guy, trying to stay above your opponents until one of them does something dumb so you can kill them.
