Keg, the 777 goes into Australian licences as 777/787. That isn’t a guarantee that casa will allow mixed fleet flying but perhaps an indication. The ATR 500-600 debacle however is the exception that might also be an indication.
Aerial perspective, the Airbus is not a Boeing and there appears to be a vast gulf between the two camps - Boeing pilots and airbus pilots - which many can’t cross. The reality is they are both excellent aeroplanes, just different.
The original, ex EK, 330’s that virgin had were fine from a dispatch reliability perspective for a new fleet. The cabin interior was one of the biggest problems, which is not an Airbus or Boeing thing, and the airframes themselves were pretty good. Like all new fleets it takes a while for pilots and engineers alike to get used to them, and people tend to play it conservatively when they are unfamiliar so it is understandable if those aircraft, as the first airbii (is there a plural of Airbus?) in the fleet, were sometimes delayed.
Had Virgin elected to replace the interior with a version of the one in the new build aeroplanes, it would have resulted in I think an improvement in perceived reliability (and an enormous bill but that is a different discussion). A lot of the problems with the two original airframes were cabin issues, IFE, seats etc.
The 350 is a spectacular aeroplane. Knowing the 777 I have no doubt the 777x will be also, certainly the current 777 is a brilliant and incredibly reliable machine - Boeing certainly got it right, but I hope the new ones are quieter inside and get up a bit higher than the current one, which is the turboprop of the jet world when operating at gross weight.