I may not be much help, as we started using pocket nappies when
DD was 4 1/2 months old - (I tried not to overestimate my abilities as a new mum and went disposable till I felt I might be able to cope!!!).
Tanya is right, when the poo is more solid they kind of slide off, but newborn poo prob won't do that and would leak out of the confines of a liner anyway.
I do use liners but only in the mornings, as
DD is fairly predictable these days - well I put liners in from waking in the morning until she does a poo - usually within 1 or 2 nappy changes!
I really like the natures child liners, (available on their website, but also at Babies Galore and prob lots of other places) but I'm sure there's lots of good ones.
But the key is rinsing for me -
DD can do some nasty ones and liners might only capture 90% - I rinse very thouroughly using gloves as we don't have a nappy sprayer then put in dry bucket. And as some wise people told me early on - SUN is also magical! The sun just seems to bleach away any little remnants of stain you may sometimes have after washing.
And I definitely don't want to put you off, as I haven't got any peapods myself, but do check out some of the threads here about them, I have heard from here and other sources that they can be a pain for leaks....
We use Fuzzi Bunz - they are v. similar to peapods, but from USA. But the Australasian distributors in NZ have had some probs with them lately and now are making their own - they are called Honey Child - you might want to check them out.
Oh, and regarding numbers...... again, it's hard for me to say with a newborn, but I think you are right, you might need HEAPS! I am a little on the lazy side and have 13 nappies, plus extra boosters and inserts for doubles at night. This is a comfortable number for me, but I can imagine I would have needed more if we started earlier. But do the sums for yourself, a big investment now will probably still save you over time..... And saving money wasn't the main reason for us going MCN, it's just a nice bonus!
All the best - it's a great journey!