I have no idea you need to post pictures of your space and the rest of the materials in the kitchen

The photo that you included actually shows cabinets that are hung at two different heights. I don’t think that it would look very good to have upper cabinets above the two cabinets on the left and nothing or a smaller cabinet above the cabinet (corner?) to the right. As Patricia says....we need more information, more photos, etc.

I have light oak cabinets and have successfully stained unfinished cabinets to match. A competent cabinet maker could match your doors. As a short person, I don't hardly use even the top shelf of my standard size cabinets, so I have left my soffits as "old fashion soffits" as the extra storage would be little use to me:)

How high is the space above? You probably won't find stock lines that will do a cabinet short enough to go up there... How many uppers do you have? Why not do a 2 color kitchen? Leave the base cabinets and change the uppers to new taller cabinets - the price will probably not be that different.

Light gray would look terrible with the oak. If you have white trim in your house, you could use white cabinets, but from your photo it doesn't appear you have enough space to add anything useful.

They wont look right. You dont have enough space for them, and you appear to have staggered cabinets.