Simple Fog definitely has some limitations, but let’s see if we can get it to do what you want.
First, set the Distance Density to 0.0 since you don’t want the fog to affect the sky dome and we don’t have a good way to exclude it from interacting with Simple Fog right now.
Second, set the Height-based Fog - Plane Height to a height just taller than your buildings. This will ensure that the height-based fog will cover your buildings in the distance.
Third, set the End Distance to Camera to a distance far enough out to be past your furthest buildings.
Fourth, set the Start Distance to Camera to a distance in the middle ground of your shot.
Finally, tune the Height Density and Height Falloff parameters to get the visual look that you’re wanting for your fog in your scene.
Here’s a screenshot of an example scene that I experimented with.
It’s all a bit of a hack, but I hope it will get you where you need to be until we can improve how Simple Fog interacts with other prims.