I've moved your thread into here, RSF, as it's not a Fanfic Challenge.
To answer your query, it's long been established tradition within FoLCdom that if you're going to use another author's plot or original characters, then you email them and ask for their permission to do so. Beyond the copyright issues, it's just a courtesy call, apart from anything else.
If permission is granted, most authors usually include an acknowledgement that their work contains elements of another author's work at the head of their story file. Thank them for granting permission, etc. Again, just as a courtesy.
Were an author to ask permission, have that permission denied, and then use the elements anyway, against the wishes of the original author, that would definitely be frowned upon and considered poor manners.
LabRat