APU fuel will be part of Taxi fuel and quantity is as per your company OPS manual.
In case of engine failure is it better to start APU so that bleed is extracted from it, which other wise would have been extracted from live engine and performance would have been reduced. Left fuel feed line supplies APU. If the Left side pumps are not working then APU fuel pump starts automatically. Fuel X feed may be selected on to take care of fuel imbalance only when there is no fuel leak.