I realize this is an old thread, but I do 2 gallon batches in the 6 gallon Brewmax often [did a Bavarian Pear Wheat recently]. CO2 is heavier than air, so atmospheric air doesn't get into the beer, it is just present above the surface and will take longer to pervade the beer than we typically leave beer in the fermenter. Other factors that effect the volume of CO2 is ABV -- higher percentages increase a larger volume of CO2. Racking your beer into another vessel also increases the risk of oxidation. You really have to try hard to get the O2 to displace the CO2. As long as you are not moving around your fermenter or removing the cover, you'll be fine.