If the proposed A350F can load 110 tons (5 more than a 777F) and haul it 5000NM (same as MZFW range as a 777F), they're onto a winner, as the A350F will burn in the order of 15-20% less fuel, as well as incurring lower en-route and landing charges due to it's lower MTOW.
One factor that's not being mentioned is that FX and 5X would have to buy new pallets and containers for the A350F. The fuselage of the A350F is narrower than that of the 777f. So in other words, what fits in the 777F won't fit in the A350F. I just don't see it happening.
All of the integrators operates B757s, B767s, A300s, MD-11s and 747s, two of the biggest ones also operates 777s. Neither of those aircraft have the same fuselage size, yet those integrators seem to cope just fine. If the priority is to ramp transfer ULDs straight from one aircraft to another, you sacrifice a bit in available volume in order to achieve that (e.g. B757 to A300, A300 to 777, 777 to 747. It's not such a big deal; FX and 5X operates large fleets of 767s which can only accept LD3 as centre-line load in the lower-deck, yet they manage just fine weaving that fleet into their A300, MD-11, 777 and 747 fleets, which can accept the LD3s side-by-side.
An A350F will be able to load an AMA in exactly the same way as a 777F, namely as centre-line loads. On the 747s they are loaded side-by-side, and that's hardly a problem.
I do have a sneaking suspicion that the A350F will be able to load the exact same ULD types (or pallet contours) side-by-side as the 777F. Volume wise there shouldn't be much daylight between the two either.
The only question is whether Airbus can get one or two of the major integrators (DHL, FedEx or UPS) as well as a few of the big general cargo carriers (LH, QR, EK, SQ, CX, CV) to spring for a combined order of 50 copies. If they can achieve that, the outlook for the 777F looks dire and the 777-8F will need to produce some very compelling numbers to be competitive. I suspect that if Airbus launches the A350F based on the hypothetical "-950", Boeing will size the 777XF between the -8 and -9 in order to gain a meaningful volumetric advantage, thereby compensating for the higher purchasing and operating costs. If Airbus stays with the -900 size, there may be enough (volumetric) daylight between that and a -8 based freighter to make sense.