A wood reducer to match your flooring should work nicely. If no one has one, they should be able to make one out of wood stock to match. It might cost a couple of bucks, but it shouldn't be too too astronomical.

no the slate is on order and am ripping out the hardwood floor area where the slate is going. I hope to be level with the hardwood and was looking to use one of the flooring strips to butt up against the ends of the run of boards in the hallway. I am 3/4 way down the hall now and need to define the end where it will go to slate. not a reducer, but was wondering if I should router a 1/4 inch channel in the ends of the boards I am using in the hallway and use the slip tongue strip of wood to lock the ends with the board across the ends. Or should I just cut the ends of the boards and butt the perpendicular board against them. end result should be something like this: http://i39.photobucket.com/albums/e157/mbclk320/Hardwood%20Floors/31-front-kitchentransition.jpg

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.

Either scribe the boards or undercut the starting tread riser.