Commander should be an equally good rendering in English of the Hebrew word "קצין", right?
I would say commander is מפקד. But I'm not an expert on military terms in either language.
No, people can command other people without being commissioned officers, and people can be commissioned officers without ever commanding anybody (I was)
Am I right that קצין is an officer as in the military, while שוטר is an officer only in the police forces?
Almost. קצין doesn't translate well as 'officer' in many contexts. However, the word can be used for the police, just not in the same way as in English-speaking countries. In Hebrew, קצין משטרה (literally: police officer) means an officer (ranked lieutenant or higher) in the police. A regular policeman/woman is called שוטר/ת.