How long should a cover letter be when asked to include statements of teaching philosophy and research interests?
For readability, it is certainly preferable to append.

If you feel your cover letter is a little skimpy, I suppose you might put something autobiographical in the cover letter, that was perhaps a little to personal for the actual statement.  For example, "my teaching philosophy was largely developed during my year as a VISTA volunteer teaching in ...."  (That's just an example -- your situation could be completely different.)

The cover letter is a good place to tie the threads together of the various portions of your application, to show yourself as a whole person, to project your personable qualities, and to get the reader interested in learning more about you.

I think you're right to keep it to one page.