The Synergy CF servo arms are included in the kits and are designed to mount to the stock futaba wheels that come on the futaba servos. This will give you the ideal resolutions on your futaba servos. If you don't want to run Futaba, or Outrage, ALign, and Savox for that matter as they are the same spline, You can use normal arms on JR and Spektrum servos.
Yes that is the right BEC Pro that Matt was using on his demo model.
AS for the 120HV ESC, I am not sure what you are asking. The Castle ESCs don't have a BEC in them, but you can wire the BEC into the flight batteries using the red and black power wires on the BEC PRO, I believe this is how Matt had his as I didn't see a flight pack.