1,2,3 gallon fermenter

[Arbe0]

I am thinking of making smaller brews for experiments and to just get used to all Grain brewing. It will also be convenient in the winter to be able to brew smaller batches on my electric stove.
If I get a 3 gallon carboy to ferment with, could it be used to as little as 1 or 2 gallon batches?
I am worried about too much head room in the carboy, and if it would effect how the wort would ferment.

 

[-MG-]

I wouldn't be too worried about the head space.

Any o2 that is left in there when you are filling it will get purge out as the beer ferments.

If you want to be extra safe, when taking a sample, before resealing it with your airlock take some co2 and blow it in the carboy for a few seconds as an extra safety measure.

Remember, CO2 is heavier than air and will settle down and create a 'blanket' over your beer.