Sorry to bring this old thread up, but I was having the same doubt and ended up here. And I came to the conclusion that the problem here is electrical distribution (as some guys mentioned earlier) rather than having a bleed source to ventilate the engine.
The SOP After start proceedures are different for the A320 NEO and CEO. The note
Keep the APU running to supply engine fire extinguishing
is only for the CEO models.
So having a look at the ELEC EMER CONFIG SYS REMAINING on both QRHs it is possible to note that:
For the CEO models on “batteries only” configuration, only the number 1 squib remains available for ENG 1.
For the NEO models both ENG1 squibs remain available in batteries only config.
So that’s it. Regarding fire protection you should keep the APU running when taxiing single-engine so you can have both squibs available in case of ENG 1 FIRE on the CEO planes.