I was told that the mags from a G21 will fit in there also, just that they will extend past the grip. And supposedly there is a nylon sleeve or something that you can slip on so that the grip is even. I think the capacity goes up to like 17 rounds.....I'm not too sure, but this is what I heard and am seeking out.
You can use Glock 21 mags with the Glock 30, and you are right, the mags will stick out. I know nothing of the grip adapter, so I can't help you there.
The Glock 21 magazine holds 13 rounds, with a +2 extention, you can get fifteen. I am sure there are other extention makers, that would add more rounds, but that would look a little funny sticking out of a G30.