Actually Soda, it was your aircraft analysis website that got me to thinking about it. Up until then, if I took a 109 it was the G10 for interceptions and the 109F for short-range dogfighting.
For the G10, I've used the 30mm with the 20mm gondolas and 13mm cowl guns to great effect. So long as I stay fast and don't try to yank the plane around, the "gorilla package" works pretty well. The downsides, of course, are that the plane's handling is impacted negatively and the shells for each gun have a different trajectory so you're not really sure if the hits are from the 13mm, the 20mm, or the 30mm. I think the best plan is, if you're going to bring the 30mm, you might as well bring the 20mm gondolas, too, just to make sure you can hit with effect in those twisting snapshots. For bomber intercepts, the G10 or 190A8 is the plane to have.
With the G2, I'm going to stick with the single 20mm and 7.7mm's, much like I did with the F4 (though I did bring gondolas in the F4 from time to time, in case I had to engage a heavy bomber formation, but most of the bombers in AH can outrun the 109F so my opportunities were limited). I think your advice about that is pretty accurate, but I've found that having the gondolas allows me to bring home more kills with less reloading. On the other hand, getting the G2 in close so I land more shells kind of eliminates the need for more ammo.
About the best advice for the G2 is to leave the gondolas in the hangar, but bring the tracer rounds even if you're a crack shot. Spitting a few 7.7mm rounds over the escaping target's head will usually convince them to turn and allow the G2 to gain closure on the angles. The 7.7mm bullets also seem to be a bit more predictable for judging their trajectory at long ranges.