I'd buy one of each cover and see how you like them on your bub... different brands fit differently and what might fit my little man, may not fit well on your bubba. The brands you mentioned are all really good IMO so you can't go wrong there. As to sizing, many people do not get much wear out of the newborn sizes and start off with the smalls (they soon grow into them if they are a little big)

As to how many, I got by on 4 PUL covers in the small sizes, but 6 would've been better. 4 is fine now he is a little older tho.

Have you looked into using prefolds on your newborn? They are a lot trimmer on a tiny bub and are just as inexpensive as the terries.