I see nothing wrong with using FB for the new baby. The problem is that the preemie size that really fits a nb well is TINY and will only fit for a very short time. The smalls though are HUGE compared to the preemie size.. they are 6 inches wide in the crotch (which i WAY too wide for a nb IMO) and I REALLY doubt they'd actually fit a 7lb newborn well. They also don't seem like they would fit for long at all because most babies will outgrow the rise in them pretty quickly. Soooo.. if you go with all FB for that stage, you'll have to choose between the preemie and smalls together or going all smalls. The first being very expensive since you'll need 30+ in each size (well.. depending how often you wash. My nb went through 10-12 diapers a day and I washed every other day so 36 was about right so that I had diapers available while the dirties were in the 4-5 hour long washing and drying process or if I was too exhausted to wash diapers) and the second being frustrating because the smalls might not provide a good fit for a few weeks.
I think a few FB to fill out a stash in the first couple of months could be nice, but I wouldn't personally have an entire stash of them for such a short time use. I'd wait until the baby fit a size medium to invest in an entire system of them. I've been nb diaper shopping and the average resell of a preemie and a small FB is $8-$10. That means for every diaper you buy, you'll lose $5-$7. Which is okay if you use them for 7-18 months. But for a 1-3 month time use, that's pretty expensive. You get much more value in the medium and large sizes.
I find that for that itty bitty stage, a fitted and cover work better at keeping poopy INSIDE the diaper
since they poop about 7-8 times a day if they are breastfed. I like kissaluvs because they fit for a long time and by the time they outgrow the kissaluv 0's, the medium FB should fit well. Kissaluv 0's are easy to find used and they have excellent re-sell value (if you pay $10 for them, you can usually sell them used for about $7 - $7.50 each) I hear KHW diapers fit for a very long time but they are difficult to get ahold of. I"m sure others have suggestions for a nb stash as well and I'm positive there are some out there that use FB only for all stages.
Keep in mind, Im not a HUGE fb fan.. I think they are an okay diaper but the fit was never that great on my dd and if I had an entire stash of medium FB only, at about 17lbs, I would have had to get an ENTIRE new system of something else because they didn't fit her thighs for about six months. By the time she was 22lbs, they fit again for a little while. And then, when the mediums were too tight again in the thighs when she was 24lbs or so, the larges were SO huge on her that she would literally pull them down to take them off and the butt sagged so much, I always thought she pooped. So, again, if we used theme exclusivly, I would have had to buy an entire stash of something else to get us through the "medium is too small but large is too big" stage (and no the petite toddler didn't fit either.. much too small in the thighs). So, I've always been thankful for a stash that was a little diversified. A dozen of this brand, half a dozen of this brand.. etc.. That way we were never caught having to make a huge investment in a different brand because what we had didn't fit right at the moment. Just something to think about. Every baby is different.