The Trackstar 1S ESC Im using have a built in 6V BEC.
The Hobbywing 3.1 1S ESC have a 6V 3A BEC built in.
Most modern ESC capable of running 1S SHOULD be able to drive a servo at 6V for 1/12 no problem. I would suggest looking up the specs of the ESC you want to use and see if it has 6V BEC built in.