do you only plan to use them overnight? i have 3 night nappies with one child so 6 with two would work though you may find your newborn is more likely to do a poo after a feed during the night so you might need more.. i am pretty sure you can boost any kind of nappy. i dnt know much about one size AIOs the ones i use come in small medium and large i think its generally fitteds and pockets that have the one size nappies..