Are the doors stain or paint grade? Are you planning to have your rooms look more open up? Sorry, need more information about your design vision. Lighter colors than the red oak will open up the room more.

Use poplar if you are painting your trim.

It depends on which floor. Red Oak (in it's natural form) can have a cinnamon/pink appearance. Hickory is gold/brown. For me, they are VASTLY different undertones.

Fir and pine work well with Hickory. Red Oak works well with Red Oak.