welcome back to NC
you can get heavier weight bamboo terry to use as flats, and around 60cm square works well sizewise. I use the bamboo terry for prefolds also, and that seems to be more durable than the single-layer flats, but does take a bit longer to dry (because it's twice as thick)
I found that padfolded boosters in a cover worked ok when it was just little wees, but big breastfed poos almost always ended up in the cover. The cover usually kept them from going further, but you do go through more covers doing it that way, and sometimes the cover ends up quite wet as well if the wee isn't absorbed in before it can run off the sides of the pad too.
hth