Cool. Yes, thats right and is what I would personally recommend doing with the BECs. Most Escs are only rated to give you 1A or 1.5A to the receiver, and the 20kg servos I've tested generally pull in the region of 3.5A - 4.5A.

Probably just best to experiment when you get the batteries, 1000mah will be more than enough but duration it does will vary massively depending on how agressively its driven!