Bah, I don't think it will be that hard once I get my questions answered.
Everybody knows the "Doe, a deer" song. The difference between that and the original hexachord is that the current one uses all 7 tones of the diatonic scale, and the original only used 6 and went "ut re mi fa sol la" instead of "do re mi fa sol la ti". Also, the current use of solfege is altered so that "do" is always the tonic - adjustments are made for minor keys or modes with the alternate syllables (minor third is me instead of mi, minor sixth is le instead of la, etc.). By contrast, the original system apparently was so that the one semitone in the hexachord (first 6 notes of a major scale, essentially) was always between "mi" and "fa". So, if you're singing a chant in Dorian mode, the "tonic" (which I'm not sure was a very solid concept back then) would be on "re". Since they only use 6 tones, it ends up sounding closer to pentatonic music than what we're used to.
Anyway, we pianists don't get much exposure to medieval music, or even Renaissance music. Keyboard literature kicks in around the time of Bach, and by then functional harmony as we know it was established. I'm just interested in this because I want to learn as much as I can about the evolution of harmony between, say, Pythagoras and Bach. That's a really weak area of knowledge for me - even weaker than my knowledge of the evolution of harmony in the 20th century.