With 5-6 lb. babies, I would also get some preemie pf and smaller covers. Bummis Super Brites should fit 4-9 lbs, or Proraps preemie. Mine have all been early/tiny and we ended up having to use disposable preemie dipes a couple of times ($$$$) because of not being prepared with preemie sizes. We are TTC #4, and I'm already planning diapers

. I plan to have preemie and nb size prefolds and covers. Mine have tended to grow pretty quickly, but I'm sure we could have used preemie pf's for over a month.
If you want to use OS diapers, (which I haven't; have always done prefolds), they will probably begin to fit (they'll be bulky, but I don't mind bulk. It's cloth!) around the time the NB prefolds are outgrown. That would be the most economical way to go about it if you don't want to continue to use pf's.