Its safe for the tube to use the same cathode resistor,as long you have a resistor tailored to the highest voltage.
But you aint get the best sound out from it.
You probably get a little earlier breakup, but thats what you want. I dont believe it's gonna be much lower volume.
If you switch to lower voltage, and a 250-270 ohm, cathode resistor and use a pair of 6V6's i believe you get a better solution.
It give a very big volume difference if you use a smaller speaker. I have my ampheads combinated with 2 - 3 different speaker cab combinations. It give me possibility to choose gear after situation.