[nationcrafting]

>The fundamental problem is you can't have a partial union.

I imagine you're talking about the monetary union, because it is of course possible to have partial unions in the EU (many EU countries are not in the Eurozone, and many non-EU countries are in the EEA or in the Schenghen zone, enabling free trade and free movement of their citizens in and out of the other Schenghen countries).

But even in a monetary union, you can absolutely have a common currency without a common fiscal policy. A currency is just a measure of account, a medium of exchange and a store of wealth. We've had centuries of common currency in the form of gold equivalence (i.e. a hard gold standard). The moments we got into trouble with gold currencies are not inherent in the gold standard itself, but in the attempts by sovereigns to jiggle around with it (seigneurage, mixing gold with other metals, creating pretend gold-standards that allow them to create more paper currency than there is gold backing it up, etc.)