I see that the Pixhawk can support up to three in flight power / redundancy options. So one output from the ESC to the output rail, then if that fails the power module will take over (ok other way around). Plus a third method by using some kind of splitter for the rail power (not really looked into that)
I am not sure I follow the logic in the power redundancy suggestions for the Pixhawk. I get that the power module (which should the be primary source of power for the Pixhawk as I understand it) will stop power spikes from digital servos (which would cause the Pixhawk to reboot mid flight). However, if the primary power source fails, ie the power module, then the ESC will not take over, because it too takes its power from the Power Module.