Stream levels can be of any shape and size and can be rotated inside the persistent level if needed.
Since Instance Meshes are part of blueprint, you will be using them duplicating Geometry per building inside the blueprint. Essentially for each building you will put frequently repeating elements as InstancedMesh. Once you familiarize yourself with some of the UE4 concepts and watch those tutorials these and more questions will be answered. I would recommend going with YAGNI (Ya Aint Gonna Need It) approach: Create the whole level by importing your assets in UE4 and look at performance. If its not acceptable optimize LODs and material rendering. If still not acceptable Break it up into sublevels and stream them in based on need. Then use instanced meshes on repeating elements, where your buildings will become highly optimized Blueprints. With what you described I think you will be good enough with optimized LODS. Latest UE4 will even create automatic LODs with some setup (look that up too)