My daughter had new #1 unstained, site-finished, red oak installed in her kitchen. Her kitchen opens to her den and dining room. Both of those rooms had 90-year-old red oak which she refinished a few years prior. A threshold was installed between the dining room and kitchen where both rooms' butt ends adjoin the threshold. No threshold was installed between the den and kitchen where the length of the board in both rooms adjoins the two rooms. Six months later, you'd be hard pressed to tell the difference between the new and old floors. All of her floors were finished with an oil based poly. You could use a different width if you insist, but your home will harmonize better if you are consistent.

You have site finished hardwood. That's the easiest hardwood to match. Simply install the SAME SPECIES (same width of plank, same "cut", same grade) and have the same wood flooring professionals come out and mix your colour to match what they did previously. If you don't want the same colour, then choose another colour for the new part. But the most important thing is to match the WOOD! That way, in the future you can have all the same colour if you want it. There is nothing more frustrating that trying to get two different species to look "the same" when adding a stain. Many have tried...and all have cried because of the failures. Personally I would have had the wood installed at the same time you had your hardwood floors refinished. That would have made every thing the easiest/cheapest. There would have been no concerns about colour matching.

Hardwood floor without a sub floor added to the joists needs to run perpendicular to the joists so you need to know what is under the existing hardwood and posting an actual floor plan here will be a good way to get answers.

M., sorry, this isn't mine. just using some pics to show you examples. the 2nd photo is a red oak, completely sanded and done w/a natural finish. You'd be surprised at how beautiful old wood floors finish up! Call in a professional wood floor refinisher and get some estimates. it won't be an issue to match your existing solid wood flooring. https://www.svbwoodfloors.com/blog/wood-floor-faqs-whats-the-difference-between-a-lace-in-and-a-header/ here's a video https://www.youtube.com/watch?v=RDqi5NAnTKk look at this before and after https://adamsfloors.com/installation