Teleharmonium,
Could I take a 7th chord(s) from one key and use them with 7th chords from another key and still develop an interesting melody? This might seem like a stupid question since I found out that staying strictly diatonic can be limiting.
Hmm, I'm not sure if I'm following you, I'll try to restate to see if this is right - you're asking if diatonic 7th chords from one key (let's say C), or those notes as intervals for an improv solo or written melody, will work over diatonic 7th chords built from the major scale of another key (say F or Bb) ?
If that's what you mean, then sure, at least sometimes. None of these things are absolute, but that's definitely one way to come up with some different sounds. There are different kinds of constructs or ways of thinking about that sort of idea.
One method is to base your writing on a more complex scale that has both major and minor thirds or both flat and perfect fifths or minor and major sevenths in the same scale. Then you can take liberties with the melody while still having a certain gravity back to the diatonic chords.
Or you can just use chords that change keys or become dissonant or ambiguous in a way that diatonic key no longer describes them, while continuing to play fairly straightforward melodies derived from simple diatonic arpeggios over them. It's like putting sweet on top of sour.
Or you can do superimposition of one chord over another, also known as polychords. Bitches Brew by Miles is a well known example of writing that is full of polychords. Another well known approach involves stacked fourth superimposition which you'll hear on "A Love Supreme" by Coltrane. Sometimes Coltrane starts to emphasize the next chord, or one 4th above the chord the rest of the band is on (which itself may be a chord comprised of stacked fourths), on that and other pieces of music from that time period.
There's a recognizable effect from harmonies that move through cycles of fourths and fifths that gives an interesting moving, searching quality to the sounds. "Hey Joe" is based on that, the basic progression of the song is nothing like a typical diatonic progression and is instead based on descending fourths (which can also be seen as ascending 5ths, except that the chord voicings played in that song in the real world do make it sound more like descending fourths).