I don't know if the 2/3 piece is necessary but I think a slightly larger topper is a good idea because you should have more control over how your sides look. Also it should be easier to blend than a smaller topper since the sides and back will be cut a different length.
I ordered a polyskin, french base, swiss front with a pretty thick density from Toplace- I should be getting it in 2-3 weeks, then I'll take the plunge. The stock piece I have I'll use as back up. In the meantime I'm thinking about placing an order from Coolpiece or Final Hair to have another back up- I really need one as I look terrible bald, it's not an option for me. I've actually been impressed with Coolpiece so far- their customer service is really good.