You need to do a little more archeological digging still. You need to know the whole construction of the floor before you can decide how best to install something new. I'd take up that plywood piece in the kitchen and see what's under it. It's higher than the surrounding floor, so needs to go. You should also be able to see from there what subfloor is under the original kitchen hardwood. It's too bad the space under your entryway is finished, but I'd still do some exploring from below to see what's under there, and to figure out the fix to stop it from squeaking. Is it drywall below or a drop ceiling? I know it probably seems like a pain to tear out drywall, but if you cut out a clean patch, like 2' x the width of the joists, it'll be relatively simple to repair. Just make sure to cut it down the center of the joists so that there's room to screw the patch piece to it later. Cut the hole under the area that squeaks the most, if you can. Have someone walk on and watch the floor from below to see what's moving. It might be as simple as pounding in a couple shims from below, or face-nailing a loose board from above. You want to find out what's underneath the hardwood. It might be laid directly on the floor joists, or there might be 10" wide boards that the hardwood is nailed to. Another thought is to take out a board in the hallway, since that's not original and you want to replace it. See what the subfloor is there. I am a bit puzzled at why, when you've stripped the kitchen down to the original hardwood, it's still higher than the hallway floor. Is your current surface perhaps not the original kitchen hardwood? Peeling back the plywood area will help determine that. BTW, I haven't heard of that stop squeaking product you linked to, so can't offer any personal opinion. And don't pour self-leveling compound over top of hardwood. It needs to go over plywood.

Sashasmommy, I briefly googled Smart Strand and it seems like Dupont is pushing it as an environmentally "friendly"/"green" product. They do list stain resistance and softness as attributes. Does anyone else have anything to add on this newer product? valinsv, you have very good points about allergies. It does always creep me out to think about the dust and the nasty mites that are along the baseboards and other places the vacuum can't get to very easily. I don't know anything about engineered hardwood. Is it as durable/thick as the other kind (I don't even know what the old kind is called...serious knowledge gap here!) amyrsq, I have a really hard time with carpet colors. I would want something that remains neutral enough that I wouldn't be tied to a certain color scheme. I guess when I think about different carpets, I have memories of seeing homes where the carpets varied wildly from room to room (think primary & secondary colors). It was jarring, shocking and incredibly disturbing to the senses. Not to mention tacky. I don't know that I would have the decorating finesse to carry it off. So, that is why one color may be a safer bet for me.

When it comes to hardwood, trends are something to ignore! Why? Because hardwood is pretty much a lifetime purchase, and unless yours is damaged in some way, you're probably not going to replace it. So, light wood vs. dark wood, wide planks vs. narrow planks -- that stuff's all going to come and go. With that in mind -- as well as the very real concerns about cleaning dark wood -- I'll vote for a nice, neutral midtone every time. Not too much contrast, not too red, and (unless it's a beach house) not too pale. I think this is the wood that's most likely to give you good service and stand the test of time.

Di you check with the manufacturer? The manufacturer of my flooring had specs and recommendations for glues. My installer said that all installers have their preferred products, so I used something that the manufacturer recommended and that he was also familiar with and comfortable using.