Is it correct to say that I discovered that they don't know? (present tense after the past tense?) In principle, no. The sequence of tenses requires the past follows the past in the subordinate clause. One of the exceptions is when an unchanging situation or universal truth is expressed in the subordinate clause.
also when i want to say that : he said that the meeting that had been scheduled in December has been postponed ? (can I use past then past perfect then present perfect? When a specific time is mentioned, the simple past will suffice. Again the passive past perfect shall be used. “He said the meeting that was scheduled in December had been postponed.”