Understanding how to keep a Multi-Building project small and organized led me to the following conclusion:
a) Build each building in AC separate and build the site separate. This works to keep the model data small in AC and it is standard practice AFAIK.
b) If you need to assemble a site in AC, with the buildings dispersed among the site, for example multi-family housing, condos, etc. Beware that you will quickly exceed the limits of your OpenGL graphics card, ram, processor. AC does not handle overwhelming amounts of polygons very well. If you need to add entourage such as cars, trees, etc. Again look out because your screen regen performance will quickly slow down to a crawl.
c) Now that the model has grown, Cinema 4d becomes the first good alternative to AC. Upon exporting each building from AC, they come into Cinema 4D with their reference to the Project Origin as it was in AC. This is why it would be good practice to build each building at points distanced from the origin and from each other. Space them out according to a pre-planned grid, away from the origin and so that they don't overlap each other. Now, allow each building export from AC to import into C4D right where it will, at the points planned in the grid for the project. After importing, make an INSTANCED DUPLICATE of each building type -(complete container file). You can then move the instanced copy anywhere you want, (x,y,z). Populate the imported site, which should be built at the center of the origin in ArchiCAD. This will create a small model file in C4D, which will load, save and store using a small amount of space.
d) The advantage of this method is that once you make changes to the individual buildings in ArchiCAD, you can just re-export a new .ac4d file and update the original matching container file. Now all instances will update throughout the site. Instanced copies will decrease the model size in C4D. You can turn-off the display of the original buildings, which will be at points distanced from the origin. You will only navigate and render the original site and instanced buildings.
e) Add all entourage, cameras, lights and sky objects in C4D and place them outside of any container files. This way they will be unaffected by changes to the building model geometry.
p.s. - One caveat, instanced buildings will not save on RAM or rendering speed. As far as these two computer functions are concerned, they "see" all of the polygons - real or instanced - they think they are the same thing. The savings are on bandwidth, on and off the hard drive, across the network and with updating the geometry with the latest version of the building design.