I’m no help on slouchy vs sloppy, but on the sewing with double gauze: it’s trickier than lawn or quilting cotton, at least in my opinion. It doesn’t stretch exactly, but it has way more give than most wovens, so you have to be careful when cutting and sewing in a similar way that you do when working with knits. It also doesn’t press all that well, especially narrow hems. The seams also get bulkier than you’d think – I needed to stuff fabric under my foot behind the needle to sew across seams.
That all said, it still sewed up fine on my machine (I do use a walking foot, but I use it for everything except zippers). I just had to be more careful, and in some cases go slower, than I would with something like lawn.