I don't always agree with the Meloncholic; It's a crude over generalization, and just because someone appears as meloncholic, like Linus, doesn't mean that they're automatically an ISTJ or whatever.I look at everything through the Galenic temperaments, and then translate it to MBTI through the Keirsey temperaments ("conative": action skills) and Berens Interaction Styles (social skills).
ESF/ENP (extraverted, informing); SP (pragmatic, motive): Sanguine
IST/INJ (introverted, directing); SJ (cooperative, structure): Melancholic
EST/ENJ (extraverted, directing); NT (pragmatic, structure): Choleric
ISF/INP (introverted, informing); NF (cooperative, motive): Phlegmatic
Charlie Brown: I would agree as ISFJ. As "wishy-washy" as he may be (from the Behind the Scenes style) he does seem to have the concrete structure-focus of the SJ. Like when he manages the baseball team. Even though he is regarded as a poor manager because the team always loses; he still gets serious and orders the others when in that position. (even yelling at Lucy, of all people, whe she's doing something silly).
Other than that, I would go with INFP.
In any case, introverted, informing and cooperative.
Lucy: obviously Choleric (In Charge). So either ENTJ or maybe ESTJ. But I don't think she's really a cooperative Guardian, so most likely the "Fieldmarshall".
ESFJ [SanMel]? NO WAY!!!
Linus. Melancholy (Chart the Course); doesn't really need people, and probably also Guardian. That would suggest ISTJ. But then he could be informing, which would be ISFJ. (He and Charlie Brown seem rather similar in type). He might seem more "intellectual", but he does not seem "pragmatic" like an NT at all, but rather cooperative.
Sally: Get things Going, which is informing, but with a bossy streak. ESFJ or ENTP. Probably the latter, because she does seem rather pragmatic, and fits the ENTP's rambunctiousness, especially in some classroom scenes I vaguely remember.
Schroeder: musically artistic pure Melancholy. (ISTJ) Leave me alone; don't bother me (when playing Beethoven). In Keirsey's theories, artisticness is associated with SP, but in older temperament theory, it is associated with the Melancholic. The ISTP, combining both temperaments is probably the most artistic, but I don't see Schroeder as being pragmatic and motive-oriented.
Peppermint Patty: In Charge, but does not seem particularly structure-focused. So perhaps ENFJ or ESTP.
Marcie: introverted, informing, cooperative and motive focused: INFP.
The old Patty and Violet: perhaps INTJ's, they way they used to just tear into Charlie Brown, and often for nothing at all. Lucy is the one who gets the bad rap, and even Peppermint Patty sometimes, while the other Patty and Violet are largely forgotten about, and basically dropped out of the series in the late 60's, when the new Patty, Marcie, Woodstock, etc. came along. But both Lucy and Peppermint Patty are simply very expressive. They would jump on Charlie Brown when he did something, like lose the game. Those other two were not as initially expressive, yet poured all this venom for Charlie Brown a lot of the times, particularly in the early 60's comics (which I grew up with in the "selected comics" books, and now have in the exhaustive "Complete Peanuts" series).
Snoopy: He's a dog. But then Pavlov did assign temperaments for dogs. He would obviously be "Lively" (Sanguine). A conation temperament and exact MBTI type would be harder to assign, since that I would think is more "human". If you must, then perhaps ESFP.