So from what you describe I assume you are running nitro with an RX pack then? Yes, your setup sounds correct, and yes the servo can have 7.4V constantly. I would make sure to use a 3S lipo RX pack, since switching BECs have a 1-1.5 dropout voltage, meaning that if you want to run 7.4V constant you don't want the voltage of you battery pack to drop below 9V (1.5V above where you want the voltage at).

I am going to try running 7.4V directly to a FlySky RX tomorrow, maybe they can handle it?