Hmm-- interesting, though I do have to disagree with a few of your classifications. I agree with Beamer that Farnsworth is more Choleric than Melancholic, though he does have the traits of both; then again, I don't think there is really any character that fits one classification perfectly without any overlapping onto another.
Phlegmatic seems to be the most contented and fulfilled of the temperaments, which I don't think describes Fry at the beginning of Futurama, but as the series progresses he does become more calm, reasonable, thoughtful, etc. I'd say that after he develops a steady romantic relationship with Leela, he becomes a strong Phlegmatic with some Sanguine characteristics, but before that I'd say he leans more towards Sanguine. Fry's thoughtfulness does vary from episode to episode though, so a consistent result might be more difficult to find than for the other characters.
Farnsworth: Choleric for reasons Beamer listed (and I am too lazy to restate, not that it's needed).
Leela: Choleric with Melancholic characteristics: moodiness, seriousness, self-reliability, and somewhat occupied with cruelty (particularly that to animals) in the world, but her general temperament is more Choleric-like.
Nibbler: Phlegmatic. Nibbler only speaks when he has something important to say or he is distressed by something, leaving me to believe that he is normally quite content with his life. He is reasonable, private, consistent, etc.-- and while he is serious and sometimes suspicious when speaking, he does not take any measures to largely change his life, despite being clearly capable of doing just that-- leaving me to believe he is fulfilled.
Aside from those, I agree with you fully-- very interesting topic we've got here.