Question: I am building a medium-sized printer which needs to produce super-precise parts at a moderately fast print time. I frankly don't want to deal with belts or their tension issues but on the other hand, having ball screws on each axis will increase inertia...right?. I'm using Rexroth rails and will use (depending on what I decide) name brand belts or name brand ball screws.

Answer: 
The size of the nozzle usually isn't the main factor for how close you can put nozzles together. To keep the filament drive gear system from being the limiting factor, you would need Bowden extruders. "Then, the heat sinks and fans would be your limiting factor. Have you considered a single nozzle with three extruders? Otherwise, you need custom angled heat sinks similar to the three heat sinks on a single nozzle, and still a way to orient the nozzles at the same Z-height. That would be difficult if all the nozzles are on the same heater block. It still seems that nozzle size is the least of the issues of putting nozzles close together.

If you search for smaller nozzle sizes, you will get nozzles with smaller openings, not smaller overall size. The threads on the nozzles are a standard size. Thus, the smaller opening size can't be put closer together than the larger opening size. Otherwise, you have only small variations between different types of nozzles and need room to screw them in to the heater block if you put all of them into one block. You can get a nozzle using a 6 mm hex wrench that is smaller than one using a 7 mm hex (E3D).
One of the thinnest hotends I've seen are those from a Chinese factory Mellow Store, the heatsink is smaller than the top flange to mount the hotend. I don't know the quality of these hotends, the image below shows the basic layouts of available options:

[![Mellow Store custom hotend](https://i.stack.imgur.com/MsvUL.jpg)](https://i.stack.imgur.com/MsvUL.jpg)

Ultimaker uses high quality hotends which are pretty small 2.85  mm hotends they call "cores", they house 2 next to each other and use a lifting mechanism:

[![Ultimaker 0.8 mm core](https://i.stack.imgur.com/pl0mt.png)](https://i.stack.imgur.com/pl0mt.png)