I've got several questions that I'm hoping the experts will chime in on:

Background

We're DIY installing about 1,000 SF of hardwoods on the 2nd floor of our home in a hot and humid area. We prefer a darker wood and have budget constraints, so it appears an engineered exotic (either Brazilian Walnut or Brazilian Cherry) will best meet our needs. DH is a professional remodeler and is very capable with lots of tools and general construction experience. The subfloor consists of 3/4" plywood with a second sheet of 1/2" plywood on top. It's glued and screwed very tightly and has no perceptable 'bounce' at all, though one small area, roughly 2' x 2' has some 'squeak'. We have the following questions:

1) We were told that thin wood products (3/8 or 5/16") require an additional layer of 1/2" plywood to arrive at a 'sturdy' floor. Is this true? And if so, do we already have it?

2) Since all of the rooms are fairly small, the needed expansion room against the walls should be fairly small and easily covered by the yet-to-be-installed baseboards. I'm thinking that for this reason, we won't need quarter round except in a few areas where the flooring butts up to built-ins, thereby saving a lot of money. DH thinks *not* having quarter round in addition to baseboards would look odd, cheap maybe; I think *having* the quarter round in addition to the baseboards makes the hardwood install look like an 'after' rather than original. What do you all think?

3) Noise transmission is a factor since several of the spaces are above our kitchen. We've decided against a floating install because of sound issues, but are debating between nailed/stapled and glue-down. DH feels nailing would be neater and easier than gluing, and avoids the off-gassing problems. Is nail-down as good as glue-down for sound-deadening? And what type of sound-proofing underlayment would work best for us?

Thanks in advance for any help --