My son started wearing only small fuzzis (old style with pointy tabs) at 3 weeks and he was about 10lbs then, though they did fit more or less from birth (he was 8lbs 13). I think they are fabulous for him and got rid of all my xs nappies, which I only used for 2 weeks even thought they would have still fitted him, they were just not near enough absorbant to last any lenght of time. i still use muslins on him (2 at a time) every so often, as well, under a fleece soaker, but mostly just fuzzis. at 5 weeks we need to boost the stuffing with something other than MF as he wees loads, but they can take the stuffing, and they aren't as bulky as other pockets i've tried on him, fully stuffed.
at night we used small sandys (boosted), or bamboozle stretchies (size 2) or size 2 flexitots (v2), with fleece wraps. of those, i think the flexitots are superb and will be getting more.