I don't have the city house, so I can't say for sure, but I believe it is smaller than this house.
Yes, I think if you ordered the baseplate, as well as the front steps, you could build one with just the floor extensions. You would have the same wallpapers on every floor that way (the ones that are in the two kids' bedrooms come with the extension), but they are just pre-applied stickers that can be peeled off so you can have plain walls. The extension comes with a balcony, window boxes, curtains, and a staircase.