http://www.duralitebatteries.com/accessories.html

Scroll down under assecories and you will find a tail servo step down, that will solve your voltage problem.

You really dont need a digital tail servo to get a gyro to do its thing on an airplane. I had a Gy401 on a Funtana and you could goose the throttle as hard as you wanted on the ground and it will take off like a rocket and straight as an arrow. The problem I had was to run the gain high enough to make it effective it would start wagging the rudder at high speeds. I needed it for a helicopter so I took it out after a few flights, it really didn't make that big a difference for a torque roll but it surely helped on take off. Any of the gyros, fancy or other will run on a standard servo they just wont be as effective, something that on a plane you probably wouldn't notice as much as you would on the tail of a helicopter.

JR makes some good gyros as well but again for what you get out of it a cheap piezo gyro would probably be just as effective http://www.horizonhobby.com for the JR stuff. Also CSM makes a line of rate and heading hold gyros, click on the CSM link on http://www.rchelisplus.com