If the Federal Govt did not have the power to declare a foreign coin as legal tender, there would have been no monetary system in the U.S. As it was it took decades for the U.S. Mint to catch up.
The fact that phrases like "two-bits", "pieces-of-eight" and "picayune" persist to this day stand as a testament of Congress's power to declare a foreign coin as legal tender. Even the NYSE continued to use the "one-eighth" system until very recently.
I assure you that studying monetary history from Columbian time onward will only add to your appreciation of our Republic. Good luck.
What continues to mystify me is your belief that a country sort of inherently needs to “build” a “monetary system” by virtue of passing laws in aid of such a construction. The “magical time” of U.S. history before the most offensive of the tender laws (you must take government paper) seems to prove to me that such is not the case. What is wrong with the free market in this context?