Firstly, after asking staff for clarification, they've said that a good guide is for anything posted here or linked to from here to be rated no higher than teen/PG-13. Wikia are generally more flexible when it comes to violence, but any sexual content must fall within that rating. If the site you are linking to contains material rated higher than that elsewhere that's fine as long as you're not directly linking to it.
Secondly, if you are recommending work that isn't your own, please link to the fic or art rather than reposting it. That way the person who created it can get full credit and any comments that people want to leave can also be directed to them.