Do minors and/or non-major courses affect grad school admissions?
I don't think most schools will know or care about the nuances of the Sahana U minor system.  Ergo, do what you want.  

Since many schools don't require a minor at all, they won't worry about if you do the extra classes in history or math or basketweaving.  They will just look to see that you are a psych major, what GPA you have, standardized test scores, essay, letter.  

It's extremely unlikely that the content of those 6 diversification courses will affect any decisions.  Just concentrate on doing well with whatever you do.  And take what you like.  The major is already covering key content needed for grad school.