Q: I think George Stark/Andre Linoge/Pennywise is really Randall Flagg because he is an evil wizard/can shape-shift/wants world domination. Can I add that to the article?
A: The problem with this and any fan speculation is that it falls under original research. If Stephen King didn't explicitly confirm it, then it needs to be brought up in a reliable source. For example, while the character of Raymond Fiegler from Hearts in Atlantis was not identified by King as being a manifestation of Flagg, the connection was brought up in The Complete Stephen King Universe, a book written by Christopher Golden and Hank Wagner.
Q: When was it revealed that Walter o'Dim and Marten Broadcloak from The Dark Tower were manifestations of Randall Flagg?
A: It was revealed in Wizard and Glass that Marten was Flagg. When King revised The Gunslinger, he wrote that Walter was the same person as Marten. Later books in the series confirm that all three characters were one and the same.