A long run of 5 V cabling might not be a good idea. It might be more appropriate to use a number of 5 V supplies - aka "phone chargers" to supply a few Nanos each. Do include the capacitors.
If you are not using the USB functionality of the Nanos, you could use Pro Minis. One programming adapter could set them all up.
Now you describe
I'd be using one output from each Nano to trigger an input on a centralised Mega which in turn sends a string via serial to a PC.
I suppose you think you need a Mega to handle all the connections. No way!
If you use Pro Minis, you chain the serial output of one to the serial input of the next. Each has an "identity" (just a number will do, or in fact an ASCII character) and passes on messages received or adds its own message. The last Nano - which may or may not share the same function as all the others - generates the message to your PC.