I am looking for details on why the narrator mentions that the fraternity brothers were all "wearing exactly the same height and style collar".
Good question! This easily overlooked detail in Ernest Hemingway's short story "Soldier's Home" may contribute in more than one way to the meaning of the work as a whole. For one, this photograph from Kreb's pre-war days shows that he was once part of a larger social unit, perhaps even conforming to the point of losing his individuality. After the war, of course, he's socially isolated, and his mother seems to have made it her personal project to reintegrate him into the world of their town: he needs to get a job, go out on dates, and so on.
The uniforms of the fraternity (that's essentially what they are, even if they aren't literally military uniforms) may also been seen as a trial-run for the soldiers' outfits that these young men will soon go on to wear. The story thus may be making a comment (it certainly wouldn't be the only literary work to do so) about how the First World War ruined the lives of a whole generation of young men.
These two approaches aren't the only ways to make sense of the fraternity photography in Hemingway's short story, of course, but I hope that they'll give you a place to start. I'd be interested in hearing what you come up with in the end!