Fermenting small(er) batches in a large(r) conical?

Hello everyone,
I'm looking at buying a conical and I have some questions that relate to head space. I'd like to get the largest fermenter I can afford. Right now it's looking like I could swing a 27 gallon unit. My question is can I ferment 5 gallon batches in this unit? I think 5gallons make for good test batches. My concerns are:

1. head space and oxidation - I've read different accounts, some say it's going to fill with CO2 and it's not an issue for primary, others say leaving kraussen that would otherwise be blown out a blow off tube contributes to off-flavors.

2. Physical shape - with a small amount of beer, will I loose a large volume to the shape of the conical when I go to collect yeast / trub?

3. What about using it as a secondary? Can I hit it with a shot of C02 to fill the head space?

Any input is appreciated!

Yes it'll work, I have the Blichmann 27 and can still ferment 5 gallon batches in it. The headspace won't be an issue and yes if you feel worried about oxidation you can always purge the headspace with a CO2 blanket. As far as losing beer to trub/yeast collection you won't lose more than you would on a smaller conical and for liquid remaining I ran a test when I first got mine draining it with the racking tube completely down, I can't remember the exact volume but it was under 1L probably around .5L that was left under the racking arm

Thanks for asking the question, and thanks for answering. I've been wondering about the same thing.

Thanks for and info BlackTie! Can't wait to get mine ordered.