Refer to arxiv version? Am I breaking some kind of rule if I'm adding the arxiv link to the bibitem for an article that's published in a journal? I thought obviously not until I noticed that (link to arxiv) and (page number in journal) are quite mutually exclusive.
No.  I think giving the arxiv link is fine.  One alternative is to have two bibitem entries, one for the journal version and another for the arxiv version.  I find it valuable to include the journal reference, because that shows that the article has been refereed (so its claims gain legitimacy).  But I also see the value of including the arxiv version, particularly if the journal is hard to access.