Update, TLDR: given multiple semi-written lines of music, which line should be deemed the cantus firmus before working on the counterpoint of the other lines?
Given a "free" (?) soprano line (by this I mean not of any consistent rhythmic form, such as just half notes or just whole notes, etc) and a bass line in first species (whole notes only)--I believe, relative to one another, these linear structures would be called unaligned and not parallel--how should I go about filling in the alto and tenor voices using species counterpoint? I would like for the for the lines to be "two-against-one" with the bass line (second species)? I eventually want to include suspensions (fourth species), but i'll let that go for now.
Should my point of reference be the bass line or the soprano line? Do I need to simplify/reduce the soprano line so that it is easier to see/build intervals? Essentially, I would like to know the most logical way to develop my piece give a free-flowing melody and a bass line in first species.
For reference, here is the music as is: