Plenty of countries have had common currencies throughout history without having a common fiscal policy (I'll assume you made a mistake when you wrote "common m

Plenty of countries have had common currencies throughout history without having a common fiscal policy (I'll assume you made a mistake when you wrote "common monetary policy", since monetary policy is the very thing you give up when you adopt a currency beyond your control). Many great empires, from the Duth republic to the British Empire, were built during periods when their government had no control over their currency.