The AF has reported that the KC-10A is too large, which would imply the A330 is also too large.
Not at all. Certainly the KC-10A is 'too' large for most mission requirements - according to the figures we were given, it offered 85% more fuel in the standard scenario than the KC-135R. Whereas the figure for the A330 was 45%.
However, when required to carry passengers, the KC-10 can only carry 75 and the KC-135R only 53. In windowless 'Rendition Class' squalor.
The next generation tanker transport needs to be properly multi-role. Although offering 12% less fuel then the KC-135R in the standard mission scenario, the KC-767 can at least carry 192 passengers. But again, ol' Bubba Boeing seems insistent on 'Guantanamo Tourist Class' levels of passenger comfort.
Both the A-310MRTT and A-330MRTT are true wide body airliner derivatives. In fact they both have the same 222" cabin cross-section. Which is wider than the KC-767 by about 1 seat - so they can both carry paired LD3 cargo bins. Unlike the KC-767 which cannot. In all-passenger fit, the A-310 can carry 214 passengers, but when configured as tanker, this reduces to 57. The A330 offers the best capabilities without a shadow of a doubt - up to 111 tonnes of fuel and up to 293 passengers, so the optimum balance can be achieved for the specific mission. Plus, of course, all Airbus designs have normal airline standards of comfort and safety.
Which is why the A330MRTT has now been acquired by several nations. The A310MRTT customer nations have no 'boom' needs, hence the aircraft is the right size for their multi-role tanker transport requirements. Nations with more modest tanker requirements might consider the A400M or perhaps the KC-390 if they have a parallel tactical transport requirement.
A mix of used airplanes can do the job sooner at much lower cost.
Not really. Operating several different types has a distinct disadvantage in terms of training and support - the only advantage being that a mixed fleet would be less susceptible to a single point failure risk. The cost of modifying and certificating an old airliner is significant - particularly if you include a boom and its operator. Which is why the OmegaAir tankers don't have booms.
Hence a 50/50 split of KC-767 and KC-45A might yet be the best option for both the USAF and the politicians.