I know this was posted a while ago so I thought I might as well give some advice. Better late than never!
1. Yes, I do use them.
2. That's right, the correct size is Standard American (color code green). They just about fit, looks a bit tight and narrow, but it is the most appropriate size. Stand European and standard will be too big.
3. Yes! and no...Well they do in width and length alright. But due to the sleeves, the pile of cards is much thicker and higher and the box is not deep enough..So I just store the Dark City sleeved cards in the box. For the Base game and rule cards, I use an Ultra Pro strong cardbox box. Mind you, there's not much space left in there either.
Hope this helps!