Hi,
Thanks for your comment! It's not counterpoint, it's just four-part writing. Counterpoint has more serious rules. Parallel 6th is not considered as a voicing error in four-part writing. My goal was to avoid parallel 5th and parallel 8th in this case. I think counterpoint is not exactly the same as four-part writing. As far as I know, four-part writing includes the following rules:
avoid parallel 5th and octave
yes, do not use tritone, or try to avoid, it's true
in root position double the root
in the first inversion double either the root or the fifth
in the second inversion double the fifth (which is the bass)
avoid large leaps between the notes
if a step is bigger than 4th, the next step should be in the opposite direction
But maybe I'm wrong... So four-part counterpoint has far stronger rules than four-part harmonization.