The movie is great.
It can be interpreted on so many levels, so these thoughts are merely my own and not meant to be taken as anything but my own opinions.
1) What does it mean when they show Pink as 'Hitler' (that's the only person I could describe as what he looks like in those scenes) and what do those marching hammers mean?
I think the movie was showing the dangers of conformity and the attempt by the state (through the education system) to teach against individualism. Pink was going through personal problems, with his marriage, career, and addictions, and his mindset became a totalitarian one ruled by the authoritarian in his personality (one created in the school system to begin with). Althought he initially rebelled against authority (through his creativity), he eventually came full circle and turned into that which he was taught - through his mother and through school (society).
The hammers were metaphors for his followers (fans), who blindly follow the entertainer towards certain doom. He was able to manipulate the audience, much in the way Hitler manipulated German citizens. Many people follow entertainers so closely that they lose themselves, often dressing, acting, and trying to become the public image of the entertainer (I see it a lot with Marilyn Manson fans, for instance).
2) The scene near the beginning of the movie where they show the kids stampeding and then being arrested
This goes back to the conformity theme. Children have natural energies that are wild, creative, and often seem out-of-control. Our schools have become indoctrination centers, that brainwash the individuality and creativity out of people and turn them into mindless automated consumers. Don't ask questions, obey authority. They remove the desire to be critical thinkers and question everything, including authority. (remember how the children eventually all had the same face and were being fed into the meat grinder? This is both society and later, the military for turning men into cannon fodder.)
Rogers uses a lot of metaphors, where what you are seeing on screen can be what you see, or symbolism for much deeper social issues (as well as personal struggles).
I'm not sure if these explanations are what Pink Floyd was shooting for, but it is the general sense of my own thoughts from the many, many viewings of The Wall that I've experienced.