Here is a good article about customzing FCK Editor.
I have always had trouble with user created content using FCK Editor. Most of the time the content is pasted into the editor with all sorts of formatting styles ( mac, word, etc) and different font sizes. And not only they look inconstant in terms of the style, but also generate a lot of extra code on the page which could be a nightmare for SEO.
This can be fixed with a simple approach:
Locate the line:
and set it to "true" (default is "false").
This way any formatted content pasted into the editor will be turned into plain text. And the user and do some style formatting within the editor.