The 321 has no outer tank, so I don't think it is a big deal to go with less than 750 Kg in the outers in a 320 as long as you have enough fuel for the flight.
Unless it has a significant impact in center of gravity.
Anyway it is worth noting that in the cockpit checks we confirm that the fuel quantity is correct and the fuel is balanced, but I have never checked that the outer tank fuel is 750 kg.
I guess that if we find the outer tanks to have less or no fuel at all we can consider that as a double xfr valve failed open. By the way, according to the MEL we can dispatch with an (o) that is like nothing. No big deal, then.
The FCOM 1 gives little information, as usual. For instance, there is not much about the FLSCU. Which BUS supplies it, or them?