The problem is the amperage difference between battery types. Volts is volts and not an issue. Go with two batteries of the same type. Your plane is worth it. Another Fromeco or Troybuilt lion battery is pretty close to the same as a good nimh and weight would not be a factor. If your running out of receiver ports you can add a Wye to a servo port. Run the servo out of one side of the wye and connect the battery through the other side.

Back to channels. If you really need more and your transmitter doesn't support mixing two channels to drive a paired servo installation, A Matchbox or two is a lot cheaper than a Power Expander.

Remember the battery sequence. Battery to switch, switch to regulator, regulator to receiver.