For a newborn, the prefolds wont need boosting - as you tend to change them whenever they wake (lots). Should you continue using prefolds once bub has outgrown the newborn stage and using the next size ones; then they will likely need boosting overnight or on longer sleeps. That being said, hemp or bamboo prefolds would last a lot longer than cotton (but pricier too).

I don't know much about bamboo flats, other than that you would need to fold them (same as terry or muslin flats) - but I would assume they would be nice and absorbant and bub wont outgrow them.

You can also use your newborn prefolds to stuff pocket nappies, so they aren't wasted once they are outgrown. Though I did figure out that even if I threw mine out (I didn't!) that it was still way cheaper then using sposies for the first few months fulltime.

Most PUL covers will fit fine over prefolds, you don't even need to pin/snappi them with some covers, just lay inside the cover and do up.