General rule of thumb is that a fanfic should have the same amount of swearing as the source material.
Um, not sure I can get behind that. A lot of source material cuts out the profanities because of the audience - pre-watershed TV shows, books about young adults that're aimed at the 12ish age ranges, etc. As such, it could be in character for someone to be more foul-mouthed than they're portrayed.

Ditto on the amateurish thing, though.