"The American people are sympathetic." is correct. It would be used more as:
"The American people are sympathetic to <someone or some people's situation>"
It would refer to some specific case.
"American people are <quality>" more says that the American people always have some quality, without specifics. Both are correct.
'18 pm' is redundant. If you are using a 24-hour clock, there is no am or pm.
18:00 = 6:00 pm. "18 pm" is unnecessary, since by saying 18:00, we already know that it is the same as 6:00 pm.