you do you, BUT Walnut would be very dark. Dark floor show every hair and dust.

Why not just clear coat them and leave them natural.

We had considered that, however, our contractor had to tie into our old hardwoods because it’s an addition, and we had concerns that a lighter stain might make the blend more noticeable! We also want to avoid any fading to an orang/yellow tone and worried that it might start to look more yellow or orange over time )-:

Red oak is going to be pink/orange/yellow. Because that is what it IS. You cannot stain your way into a different species. IT's always best to roll with the flow, and barely stain any wood. That's the look that has the most longevity. It will go fine with your whites, but you really need a warmer green than EF.

I've always used Jacobean - not too dark, not too light imo - we feathered in new wood with 100 yrs old wood and you cannot see the change

Since you have unfinished floor right there, your flooring guy should put down 3-4 sample finishes for you to look at in your space in your lighting!.

Something like this: