The original question had to be edited down a bit. I think that Paul can speak for an entire generation through his own experience. He strives to take the individual's struggle and broaden it out to more individuals. Consider Remarque's opening lines of the book:
This book is to be neither an accusation nor a confession, and least of all an adventure, for death is not an adventure to those who stand face to face with it. It will try simply to tell of a generation of men who, even though they may have escaped shells, were destroyed by the war.
There are some points to make from this. The idea of war being justified as "an adventure" has to be articulated. For so many both leading up to the First World War and its continuation, there was an appeal of the basic idea that war is adventuresome and filled with a sense of anticipation. Paul's narrative dismisses this. It makes sense for him to do so because he is speaking from the experience of being at the front. His experience must be validated because of what he endures as a result of it. If it speaks to another soldier's experience, then I feel that he is capable of speaking for an entire "generation of men" that were permanently impacted by the war.
At the same time, I would suggest that Paul's narrative and his experiences must have merit. He speaks from a point of view that is different from the command who might fail to grasp the implications of their decisions. Consider in chapter 6 when Paul talks about how the inexperience of the recruits actually end up killing them because they are only rooted in theoretical knowledge and nothing more. Paul's narrative can be seen as being just in its speaking for an entire generation as no one else is speaking for them. Even if one argues that Paul is excessively negative, the reality is that the casualty count from World War I was brutally high. Someone has to be able to speak for such an experience. Paul's account details how the lack of proper training and the lack of guidance to so many young men becomes the hallmark of World War I. His voice as one of experience and one in which the need to pay attention to the plight of soldiers immersed in the hopeless condition of war is one that makes him qualified and justified to speak in the manner that he does.