|Well, really there's no reason to think that the One-Size won't fit right from the beginning. Sure it'll look bulky but you can be pretty sure that it'll work, many moms here have even started before 8lbs with no problems at all.
That said, I totally loved my xs Sandy's and would do it again if I had another and could afford it. They're a luxury for sure as the small Sandy's or One-Size should fit fine from the beginning. But they're a very nice luxury and they do have the practical side of allowing newborn clothing to fit better.
Mine fit my twins (7lbs 6oz and 6lbs4oz at birth) for about 4 months. They were tall and thin though (and even then the last 3 weeks or so we used them without snapping the leg snap). Others have reported that they only last a couple of months or even less so it really depends on the baby.
24 sounds like a good number, you might not have enough to hang to dry but if you dry in the dryer you should be fine. I think they should sell well (or conversely be fairly easy to find used if you're interested). Because you don't use them for long they're usually in great condition and people are more likely to want to buy used.