Yes, you can mix different servos to some extent. Meaning that if you have dual aileron or elevator servos they should be matched for those control surfaces. If your throttle or rudder servos are different its not too much a big deal.

Why? Because the speed and torque strength of all servos isn't the same, so its not advisable to use different servos on matched control surfaces.

Larger aircraft with larger control surfaces require higher torque servos to get the job done. The throttle servo is the generally the least important.

As far as receiver batteries if the servo is rated for either 4.8 or 6.0 volts you will get higher torque out of using a 6 volt system. Be sure to see if your servos are rated for 6 volts if you thinking of going that direction.