Answer: That's a good question and kind of a coincidence coming after the previous one. Really you could say that harmony and counterpoint are always combined, but harmony is the result of counterpoint. That is, when singers and composers first learned ways to make several voices work well together, what they were doing became known as the art of counterpoint. Harmonies were being created, but they were just the consequence of making the voices agree. As counterpoint developed there grew to be an awareness of the chords being formed by the several voices, and eventually harmony came to have a life of its own: to simplify a little, people began to think, this is a C major harmony, now we're moving to a d minor chord, and so on. Eventually progressions of "chords" came to be considered the basic harmonic logic of a composition.

But even now there are compositions in which counterpoint is dominant, where the melodic aspect of each voice comes first. An example of that would be a fugue - in a fugue the logic of the melodies is in charge, and their interplay is counterpoint. There is harmony happening, of course.

As for following rules: no one really has to follow any rules. But the "rules" of counterpoint are recipes for achieving a certain effect. If you want to get that effect, then you follow the recipe. Of course, once you become a master chef you might decide to change an ingredient here and there.