I always use 5/16" parallel fit piles for field archery, here in the UK. The bamboo shafts I use (from Mark in England, hi Mark!) do vary in diameter, but I find that if you are careful you can slim them down by using a scraper or knife on the rind at the pile end. Keep turning the shaft and the diameter reduces evenly for the pile to fit securely without cutting through many (if any) power fibres in the shaft.
I dont bother reinforcing the pile end of the shaft any more, although I have tried it with hardwood offcuts. Most of the bamboo arrows I have bounced off trees or hit rocks with tend not to break at the pile, unlike pine or POC shafts I have also used. The bamboo shafts tend to break only around the start of the fletching area, which seems to be a weak spot in all arrows in any case.
I do reinforce the nock end with a hardwood insert, and also wrap the nock area with button thread (or even red embroidery silk!) soaked in HMG. Unless hit by another arrow these nocks seem really very tough.