Are you serious? I have different wood in every single room. And each room has multiple kinds of wood. And the hallways are different from the rooms. And in my bedroom I just used all of the stuff I had used elsewhere - it's actually my favorite floor. All wood looks nice together IMO. And why stain? If you want something darker, get something darker. But keep in mind that dark woods show dirt and scratches very easily - more so than lighter woods. Something like oak, with a lot of grain, is good at hiding. Just go around and look at different woods and then design each room the way you like. The wood will all go together in the end. Good luck.

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.

Please choose another "visual" for the tile. Wood look up against wood makes BOTH materials look cheap. After spending all that money on wood floors, that's the LAST thing you want. Remember: bathrooms are allowed to have a different floor - because of the wet issues. Therefore they are allowed to have a different LOOK! That's what's so much fun about bathrooms! You can make the smallest room in the house into a show stopper! It's so much fun to design a bathroom! Why would you limit yourself to the same look seen in the rest of the house? Have fun - and start thinking about how you can make your bathroom stand out, not blend in.

The house is 100 years old which means the floors might be the same age. It is entirely possible that the wood has had its day. A wood floor can only be sanded/refinished so many times and then it is finished. It needs to be replaced. Before you decide on 'other' flooring for the house, PLEASE find out if there is enough wood on the EXISTING floor to get another sand/refinish. To do this, you simply need to find a cross section of the wood. This is often found around floor registers or at the floor's edge (in front of a door, or at a step, some place where you see a transition strip). Find a spot to look at the cross section and then MEASURE the amount of wood sitting ABOVE the tongue. You need a minimum of 3mm of wood to achieve a sand and refinish. The other thing I would like to point out is the wood is in rough shape...but the finish (just from the glow in the pics) looks OK. I'm wondering if the previous owners did a 'buff and coat' (adding a layer of finish rather than having a full sand and refinish). This might be because of resale or it might be because there isn't enough wood left to achieve a full sand and refinish. And is this part of the home and addition? If it is, then the age of the floor/addition might be to your advantage.