I don't have an article to back this up, but Schwimmer gave a talk at Northwestern several years ago in which he was asked about the lack of diversity on the show and defended himself by saying that he had campaigned for his character to have a love interest of color, hence Charlie.

Does anyone know who did that cover of Don't You Forget About Me they used at the end?

I took it as Chalky was expecting that the other prisoners wouldn't know how to read and said Tom Sawyer rather than David Copperfield because the former would be more recognizable/respectable than reading Dickens.

Madison Square Park
That is indeed Madison Square Park, I saw them filming there over the summer and have been waiting for it to turn up on the show ever since.