"Devil" certainly demonstrates the normal elements of American Romanticism (supernatural encounters, time spent in nature--especially the dark forest, and moral lessons). In regards to nationalism, Irving's short story does provide the reader with insight into early Americana. For example, Irving plays on some of the curious superstitions of the Puritans and other settlers (the devil as a being who inhabits the forest while trying to gather unsuspecting settlers into his fold), and he also satirizes the hellfire and brimstone view that many early Americans had of God and the devil. In this sense, Irving (who was also widely read in Europe) allows his readers a glimpse into the beliefs, traditions, and superstitions of the people of the New World.
As far as the Romantic quality of Individualism goes, the author specifically portrays Tom as an individual--someone who makes his own path in life (albeit a suspect one) and who really does not seem to care what others think about him. In general, the idea that individuals have to make personal decisions about their afterlife and lifestyles here on earth also fits in well with Romantic ideals.