|I, too, was able to use OS and S Airflows from 6.5 lbs with my scrawny preemie. I didn't think they'd work, but I was having so many leaks with the preemie/newborn disposables that I figured it was worth a shot, and zero leaks. As PP suggested, though, perhaps it is about baby build?
I'm sure the XS Sandys would work wonderfully for you; it's possible that S Sandys would also offer some advantage to you because of the the more fitted design. And of course if you go with the prefolds you can always use them later for boosting night diapers, etc.