Hello,
To make things clear, instead of modern major/minor 7-degree scales, old music used three overlapping 6-degree scales:
1.) normal hexachord: C ut, D re, E mi, F fa, G sol, A la
2.) soft hexachord: F ut, G re, A mi, Bb fa, C sol, D la
3.) hard hexachord: G ut, A re, B mi , C fa, D sol, E la
"Mi contra fa" was considered bad, and usually avoided between different hexachords, both melodically and harmonically: E mi contra Bb fa, B mi contra F fa, B mi contra Bb fa. (Note that C, D, G and A exist in all hexachords.)
E-Bb and B-F are diminished 5ths (or augmented 4ths) while Bb-B is an augmented prime.
So while they were bad intervals, I think 7th was not, especially when using slow (long) notes.
Here is an example of a Ionian cantus firmus outlining a major 7th (lower notes in capital):
C, G, A, G, F, G, c, e, d, c
About your second cantus... In later music (~1600-) diminished intervals and 7-degree scales became more and more common, so later theorists usually allow those intervals. They sound better in downward direction, and can be resolved by a rising semitone, as in Salzer's Aeolian cantus. Anyway, "strict rules" are very arbitrary and differs from theorist to theorist.
Good luck!
Máté
PS.: Please use cut time for Fux's examples, otherwise 2nd species won't work (no weak beats).