Great work so far.

About the buildings. For the first type I think you should
mix the both models each one is good in some way.
for exemple I love the lower part of the left building
and the top part of the right, so I guess mixing them and
adding some details would be great.

For the second type i don't think it looks like an office
building it's more like a reactor of some kind
and I would add 7-8 copies of the left one
around the power plant and connect them with some
cables and connect it all with power plant.
(If you have seen FF7-Advent Children in the begining
there is a power plant with reactors around it so that's
the style I'm talking about)

Again add details 'couse that's what makes the image come alive.
Add some building that are far away from the camera
to fill the empty space in the background.
Maybe even create some low poly models of
space ships flying around.