1) Castellano did not refuse because of the salary. His wife who had a minor role as Mrs. Clemenza in GF1 took over the correspondence with Coppolla.
http://www.http://www.gangsterbb.net/threads/ubbthreads.php?ubb=showflat&Number=558311

2) In the comment on GF2 FFC says that he more or less simply replaced Clemenza by Pentangeli. To me it makes perfect sense: The betrayal of Clemenza to whom Michael had a very close relationship would have been another piece of the whole picture: Michael loses all the people who meant something to him and/or kills them. His relationship to Pentangeli is loyalty because of his father. His relationship to Clemenza is friendship or even a nephew-uncle-relationship.
There were a few minor changes, like the black armband of Pentangeli. Even the appearance of the brother at the hearing makes sense: We had seen old Clemenza in GF, we see young Clemenza in GF2, and now we'd see his Sicilian roots.
Also: In the end, Tom says "you were always interested in politics and history. I remember you calling about Hitler back in '33." This refers to the scene in GF where Clemenza says to Michael: "You know you got to stop them at the beginning, like they should have stopped Hitler at Munich."