I don't know where this arming switch sits in the circuit so I can't say for sure. But if the current to the motor will flow through that 14 gague wire, then you have good reason to be concerned.

If everything is built out of 12 gague, they did it for a reason. If you put 14 in the middle, you create a point of higher resistance. Depending on the amperage you are pulling this could be a problem. How many amps are you expecting to be drawn through the system?

If you connect in series you will double the voltage. Will your ESC and your motor handle that voltage?

You should only do this with matched packs.