Hi, My twins were 2 month prem, and I didnt start using cloth until they were about 3 months or so. but that was more to do with I didnt want to spend a lot on super small stuff, then have to buy bigger. But Riley took so long to grow that I relented and got her some super small stuff.
Riley is still small now, only about 12lbs, and super skinny. I started her in bimbles and diddy diapers, which are perfect for teeny waists and legs. I'm now using mostly bambinex which are super soft and again good for skinny legs. I also used x-small fuzzis and real easy by hip hip baby (which I LOVE). She still isnt a big wee'er so the x-small stuff is great.
Her brother Lachlan is chunkier and is mostly in bamboozles. On the odd occassion I run out of rileys stuff I pop her in a bamboozle but its just way to bulky for her, although thats what I use as a night nappy for her.
I suppose what I'm trying to say is in my very inexperienced opinion sized ones are big on a prem baby! - but the smaller nappies don't last all that long. If you don't mind lots of bulk you could go for the smaller ones to start then get size 2's or birth to potty.
Good luck on keeping your bub cooking though