is a link to a 3 gallon waterbottle that you may want to look into. Its cheap and would really help you. If you want to scale down I definately recommend it because its cheap, you can use the spring water to brew with then use the bottle (wich is PET - same material as Better Bottles) and its the perfect size for 2.5 gallon batches.
To answer your question you can do it but its not to be advised.. with all that headspace there are numerous things that could go wrong. ( infection, oxidation, etc.) but its not unheard of or impossible to do.