We´ve got currency and a trading System on our Server for long time ... and mostly the silver coins are used.
It also doesn´t ruin the economy if one clan got´s all of the gold. Even in Lif, currency can´t be eaten! Aditional I´m sure, if the economy is dictatet by just a few players, there will be GM´s which can steer a little bit against that. Even in MMO.
For the mint perhaps the leader of an Tier 5 Monument (or Tier 4 in YO) could open a special window where a max. of each currency has to be set. After that, the moneyer can do his Job until this limit is reached (or he runs out of material).
And there should be only one mint buildable for each kingdom.
There also should be a possibility to recycle coins. (Last time I tried, I get iron lumps from silver coins).