I'll add gold mining, aswell as gold importing to the NS.
Yes, gold coating is of an important issue, gold is the mineral of the Emperor, therefor, since the Holy Legion is essentially the honor guard aswell as Royal guard of the Emperor himself they are coated in the stuff to look as majestic and Royal as possible, there's also a couple religious reasons.
I took the plague into account when numbering the cities of the Imperium, I wanted it to be a large empire and simply because of it's large population disease and plagues would be a problem, which is exactly what I planned. But if you wish for me to make the city population smaller then I will do so.
Off topic, but I just wanted to say this. What I was thinking about the Exiled and Holy legion format is that they differ incredibly in strenght, the exiled legion using only outdated Equipment for example and the Holy Legion basically being the most experienced, veteran and well equipped Soldiers in the entire Imperium. They were supposed to be infamous for their strenght and legion size, and since they use copper which is easilly re-shapeable they would also be known for their majestic appearence.