I wasn't very clear in my last post. In standard tuning, the 7th-fret harmonic is the same note as the fifth-string harmonic on the next string down (for every string pair but the G and B strings, which are tuned to a major third). In other words, a 5th fret harmonic on your low E string will play an E (two octaves higher than the open string); the 7th fret harmonic on the adjacent A string will play the same E.

As for "artificial" harmonics (played twelve frets above a fretted note), they can be done. I learned these in my classical guitar studies as a youngster, and was taught to finger the harmonic with my index finger and pluck the string with my third (ring) finger. It's a cool trick, and you can get to the point where you can do it reasonably smoothly. Other people finger them differently that the way I was taught -- some folks who play with a flatpick finger the harmonic with either the second or third finger. And yes, this way you can play a scale. If you're just playing the "natural" harmonics (12th, 7th and 5th frets on an open string), you have to settle for the notes that are there; unless you are in an altered tuning, a scale won't be available to you.