But classical liberalism has everything in common with libertarianism. In fact, they are interchangeable.
This might be the case, but it would be a mistake to characterize the Founders as Libertarians, because clearly, they were not. Maybe you could call them small-l libertarians, maybe. If you read their works and what they actually did while in office, you will see that they were NOT Libertarians.
BTW, I consider myself a libertarian, so this is not an anti-libertarian rant.
It depends on which Classical Liberal writer you're looking at as to the degree of conformity. In the case of someone like John Stuart Mill, absolutely that is the case.
The founding fathers were NOT rabid conspiracy theorists who hate the government (they founded it, after all) want to abolish ALL taxes (as if that's possible) but still uphold the Constitution of the United States, at least as it was originally written, because the Constitution is the basis of our government (which they claim to hate).