There are $5 voltage regulators that can be used, InLine, with an individual servo. I have seen them mentioned in other threads with web links posted.

I have some JR 791 Retract servos that cannot handle 6v. In my case, the aircraft is an H9 P47-150. I just use a 5v regulator between the Switch Harness and the Rx since I do not need the extra Speed and Torque for the servos on the other channels.

What type of Rx battery are you using? Is it a 2cell LiPo, 2cell LiFe, or a 5cell NiMh? 2cell LiPos charge to 8.6v. 2cell LiFe's charge to 7.2v but drop very quickly to 6.6v. I use 2cell LiFe or A123 Rx packs and I have never had any trouble with servos rated for 6v. HV servos should handle 8.6v but I realize that you have at least one non HV servo.