I bet it will be fine, I have made small batches of wine, like a 4gal in a 7.5gal carboy. I thank that as long as there is a layer of C02 on top it should not oxidized.
awesome... I'm gonna put a half batch in a 5 gal carboy someday. It'll be my first AG w/ my PM set up. I just wonder how I'll cool it to pitching temp. sheesh... guess I'll have to convince swmbo to let me get a chiller also.