They were respectful in their behavior(s).

The thing that matters here is the context. If the behavior refers to the behavior of a whole group, the it should be a singular (behavior) but if the context says that every one of them was meeting you seperately and everyone's behavior was respectful then it should be behaviors.

I knew he said i was mad.

I knew he said i am mad.

There is no grammatical mistake in the above two statements. Both of them are correct but differ in meaning. In the first example, the word mad has a metaphorical meaning - i was angry. In second, the word mad has a litteral meaning - i was actually mad.

Writers jump from tense to tense (logically) to add vividness and to give a touch of continuity. 

GB