I didn't buy any premies. We went straight to "infant" w/ the dark green serging. The folding down wasn't bad (I did it in front instead of back, since munchkin was on his back so the bulk didn't affect him). He was 8lb9oz at birth and was back to birthweight by 4 days and climbed quickly up to 12ish lbs. At 3mo I'm not sure what he weighs, etc but he's squarely in med fitteds and covers (some of them even seem small for his rise).
Preemies might not be bad for stuffers in pockets (if you include that option) b/c the infant size really was too big to stuff into a FB in those early days. (Later I got a bunch of doublers/stuffers off TP for a reasonable price, so if I use one of my rare pocket diapers, I stuff w/ those instead of PFs)