The MAX6 rated for 8S so it should do 8S. The stock motor is only rated for 6S though so you'll need to change that also.
As for the batteries the only thing about running packs in series is to make sure that you keep the batteries in matched pairs. Other than that its the same voltage either way.
Yes, chances are if your motor is running hot then your ESC probably is also. Get a temp gun so you can keep tabs on temps so you dont permanently damage something. Now if you put a MAX6 ESC in it and only Run 6S then your motor would get hot still but the ESC not as much since you wont be pushing the MAX6 as close to its limits as you are the motor. Oh, and I would definitely run cooling fans on everything.