What prop size
The 9 x 6 APC is the official race propeller.
For break-in a smaller propeller that lets the engine rev-up is best. I've used a 9 x 4 APC and 8 x 6 APC with equally good results.
All of my engines were the newer generation. You don't need to change out bearings on the newer engines. The bearings seem to hold up fine. In my experience you do need to let the engine turn up during break-in for best results.