You should operate the IMU with no more than 5V. The ArduPilotMega has an onboard voltage regulator which allows higher voltages than 5V. I do NOT recommend to operate the servos directly on the APM using the onboard voltage regulator. It's powerline going to the terminals is so thin, it will not support enough power to the servos for flight operation.
You can use a 5V UBEC regulator to power servos and APM directly at the servo terminals.
Best is to separate power for servos from the APM. I use a 8ch optocoupler module from JETI and power the servos from the speedcontroller BEC. APM and receiver are powered by a similar 2S lipo using a programmable UBEC from Castle Creation. This protects the sensitive electronic from failure of servos and other high Amp devices.
I'm not using the APM, just the IMU for a different project (GPS blimp). On the IMU it says Vin>6V so that makes me think the IMU has a voltage regulator also? Is that not true? I'm going to use a 5V regulator to power the servo now.