Is it legal to discard documents or other evidence when a request is made for the information, or when you think a request is about to be made?

No, it is not legal. While you do have the right to dispose of documents in the ordinary course of business,  if it's consistent with your policies, you do not have the right to dispose of documents after a request is made  (even if it would otherwise be consistent with your disposition policies) as it would be in violation of the  California Public Records Act.