I don't care for mixing unless the rooms are very separate. I would use the same hardwood that you already have and then have it all sanded and stained to match throughout.

I'm with Deborah. Ideally you want to keep a single flooring choice throughout a "level". Of course kitchens/baths have traditionally been "outside" this rule because of the unique moisture concerns. When a kitchen is already in a wood finish, the bathroom is the only floor that is exempt.

If you can match the species and plank width you are in great shape. If you "want" a dark floor in other areas, you can have the floor stained to your choice of colour...knowing that the two floors are identical underneath. I don't recommend this approach but if you must have 2 different coloured wood floors, it offers you the most flexibility in the future. If you decide to sell, you would then need to refinish the floors to a single colour.

For resale value...a single species of wood with a single colour throughout is the "Gold Standard" in flooring.