I'm just starting out home brewing and I am planning on sticking with 1 gallon brews for a while. I want to purchase a Tap-a-Draft system but continue with 1 gallon batches for a while. Can you load less than the full volume of a Tap-a-Draft? Would the extra air volume cause any problems?
My main concern would be with what's in the extra air volume. If you don't purge it with CO2, a couple things could happen:
Purging with CO2 will remove the O2 to prevent oxidation and prevent aerobic microbes (requiring O2 to survive, main type to infect beer) from being able to survive. Even if you don't get all of the O2 out, CO2 is denser than O2 and will sink, creating a blanket of CO2 over the beer and effectively isolating it as long as your keg isn't shook up.