A350 won't be a different fleet, it will "likely" be mixed fleet flying, as it's intended to be CCQ approved via Airbus - big picture.
Why would CX not utilize Airbus crews, but go through the effort of a 777 vs 747-800 CCQ? Two completely different aircraft?
I heard from a top source that the A350 may have it's own fleet office, but the pilots will be CCQ'd. As said, a lot can change, but the bulk of the cockpit and operation of the 350 is a 330 with advanced avionics, EFB's built into the flightdeck as part of the Nav displays, still has Trent engines, and will have no stab tank and only 2 hydraulic systems. Sitting in a 350 vs a 330 won't feel like a completely different aircraft, hence the reason Airbus have included it in their CCQ program.
Taking an experienced Airbus pilot and putting them in the A350 is a logical and ideal approach from a line ops point of view. Airbus is a handful (from a technical standpoint) as it is, never-mind the leap the A350 will be. If it comes down to money, as all in CX, and the CCQ is approved by Airbus, I can't find a reason why the company would separate the pilot group on the Airbus type. The A330E has more in common with the A350 than the A340!