headspace in secondary?

I am thinking about trying to make a fruit wine soon. A 5 gallon batch. I think I understand the process but I am limited on equipment right now. I was planning on preparing my must and fermenting in an ale pail for the first 5 days. Then I was going to transfer to a 6 gallon BB for secondary but I am concerned about the headspace left in the BB. Is it too much? Will there be enough fermentation left after 5 days to displace all the oxygen in the BB? I really don't want to buy a 5 gallon BB because this fruit wine thing might not be something I do very often. It's really just an experiment at this point.

One more question. During primary fermentation is it better to cover the pail with the lid and airlock or a towel like I've read on some websites? I think the reason for the towel was to allow oxygen to pass through for the yeast because you wouldn't be able to shake the primary with all the fruit in there. I don't have an oxygenation system. It's on my wishlist though.

Use an airlock...it's doing the same thing as your towel idea, but it's doing so without allowing bacteria inside the fermentor. With a towel, you'd have airflow going both in and out of the fermentor and that's not good.

As far as headspace...you'll be fine with a 6 gallon BB.

Monitor the SG so you can transfer it before fermentation is finished.