I love Patlabor. Your diorama idea is going to be a cool one. I can't wait to see when you finish!
My only advice would be to add more of a action pose to the labors. They seem too low key for your idea. If Griffon is getting the drop on the two Ingrams, he ought to be in a more assault stance. Arms bent and back, elbows in, hands out slightly. Show off those claws. His legs might be splayed out a bit more, and a deeper knee bend for his landing. Maybe even scrape one of the building with his leg.
The Ingram to the left ought to also have a weapon ready to bea, or reaching for it's weapon.
The Ingram to the right ought to have a deeper bend in his legs, and his left arm out and raised more.
The Hummvee is OK, but the scene is Patlabor, so a Special Control Vehicle might look better for the part. At the least, the Humvee needs JSDF markings. The figure standing by the Humvee does not add any emotion to the scene. He's just standing there.
The Huey in my opinion, might be a little too much.