You need the headspace in the primary for the krausen, but it's not needed for the secondary. Minimal headspace is supposedly best in the secondary, but I think even a large headspace is fine because of the blanket of co2 that covers the beer. I have stopped using secondaries altogther, and couldn't be happier with the results. I'd say buy the 6 gal carboy and use it as another primary. I think the yeast do a better job of cleaning up after themselves when left on the yeat cake, and unless you're leaving it there for months, there really isn't any risk of autolysis.