Wow, you've got a healthy listening list there. Mr. Simple is flawless aha. I'm honestly far more behind in the K-pop scene than you so I don't know much, and after PSY, I became less interested since if something becomes massive, I'm just not all that fond of it, to be totally honest. Just.. gets the wrong crowd and way too much attention on the wrong things to me. I think most of the viewers were looking at Hyuna throughout that song. xDD I think I like to take my time going through this particular music, because I feel girl groups some very same-y. I know of most of these K-pop groups, they just don't hold my attention long enough to properly check them out. But the ones I have done when I first got into the Asian scene I'll continue to look into, for example, 2NE1 and Girls' Generation. I've just listened to them for a while now so I've stayed heh.
I read someone say they're not liked that much. I got confused because of that since I thought they sounded better than the mega big groups. I agree, I feel like their songs are more melodious. Must just be because of which songs I checked out. I gave a little listen to their album 'Sound G' ages ago and remember enjoying it, so I'm glad you're liking them. About your question, as I said above, I have little knowledge about K-pop but it appears they became official in 2006. That album I've spoken about of theirs was released in 2009, with that track, 'Abracadabra.' And on the notes of that song, it is said it 'marked a change in the style of the girls from more conservative to a provocative one.' So, maybe not the first group to have such a sound, but it's the first time their style changed.