For the most part you are correct. If you read through the GradeSaver character list it becomes obvious that Blanche is the protagonist and Stanley the antagonist.
Any of the characters without actual names (strange man, Mexican woman) would be flat characters, and perhaps even stock characters as well. I would consider including Steve and Eunice under Flat characters given their inability to change or develop themselves.
Round characters would be the main characters who are fully developed, and I would include both Kowalskis, Blanche and Mitchell in this category. Dynamic characters would likely include this group as well.