Never even thought of using something like that. Round here I see a lot of milk crates tied to racks, but never a pickle bucket. I suppose you could carry your minnows for fishing.
As far as one pannier over two, I ride with one a lot. I put my laptop in it, wrapped up in three layers of plastic and clothing. There is a big difference in the ride, like I can't no hand with it, but other than that there isn't a big problem, now if you are talking about trying to fit the gear of two panniers in one, you might have a problem, but that is why I have the other, plus a "trunk", and of course there are the front panniers when I put that rack on.