My domino runs perfectly on 6V.
I always connect 6V from a power supply (stable) and everything (including shields) runs perfectly ;D
How much current is the PSU putting out?
1.4A max. on 6v
Well, maybe it will be ok then. The ESC BEC is outputting 1.2amps at 5V.
But your topic title sais 6V ?
It’ll be 5v at 1.2amps when it the voltage is reduced slightly.
Ok great, you can use the electrical suggestion suggested before then. 
Adding rectfier diode is the best thing to do. Reducing voltage in easy way and get the results as you want.