Initially, that was my leading concern. However, the surface area of the wort exposed to air is the same, whether you put 2, 3, 4, or 5 gallons in a 6.5G carboy. So I don't see how the risk of oxidation is any greater than a full batch.
I suppose one could argue that a smaller batch will take longer to produce enough CO2 to cover the surface area of the wort, but how much longer would that be? A few hours at most?