Neither cars or pedestrians have a intricate system. They just move along the side of the road. Some collisions might be inevitable :D  

Of course! Everything can be swapped out. You can mix and match the existing asset collections, modify them or add your own. Just be sure pay attention to object boundaries and origin points.

Chances are - you won't be able to create a metropolis. However, the better the specs your rig has, the larger the city you'll be able to generate.