Hhhmmmm- i would go the bamboo fleece for the prefolds. I think keep them fleece all over with two layers on either side of a three layered centre panel. Regarding the thickness I would go for a medium which I think is about 370gsm. I got some 280gsm I think which is really too thin. I have ordered a medium and heavy (430) bamboo fleece but waiting on their arrival.
I don't know the bamboo terry so not sure about the flats. I can measure one of my cotton terry flats in the morning if you want the size?
PM if you want to know where I ordered my heavy and medium fleece from- it was way cheaper than anything else I could find but I haven't received it yet either!