I originally posted in the comments, but since time has passed, I think it more appropriate to be shown below:
Hexachords are chords or pitch-class sets that contain six notes.
A hexachordal system refers to exactly what you outlined: use of the first six modes of the diatonic scale - which had been established long before Fux came and wrote his text. The seventh mode (Locrian) was not used because it does not naturally produce the interval of a perfect-fifth, and therefore renders authentic cadences impossible. The scale is omitted because it is essentially too difficult to work with without considerable intervention.
I should add here that different types of hexachords were employed and that you should refer to page 31 of the text for further clarification.
Hope that helps.