With regards to your SBEC selection..
Your need to create a "power budget" for your particular situation.
Total cyclic max servo current draw, rudder servo draw, RX & FBL controller draw...voltage drops across any switch...and connectors.
By far, the servo current demand will be the major current "hog".
With this "budget", you can then "size" an SBEC that's appropriate to your particular situation.
It's also a good idea to get an external SBEC that has a user programmable Vout....run the Vout to the max. extent your can...depending upon servo type.