I agree that the stereotypes were a bit overused, however that type of ethnic predjudice was very prevalant at the turn of the century. Especially between Irish and Italians.
The Major was in fact a major, and a Lawyer from NY. Back in those days, political influence and money could still buy rank. This was one of the major command problems with that war. The charaters were real people, unlike Ryan, where they were generalizations. Read the book. I believe the man who stated that he was at San Juan Hill, was a career NCO. The captain that showed up halfway through the movie was with Pershing, chasing Villa.
There's a book called "The last days of Innocence: America's entry into WWI", (I think. I'm not at home right now, so I can't quote the title exactly, I have an entire book case dedicated to the great war) that talks extensively of the social culture in america prior to, during, and after the war.