“I don’t like to mix spellings on a page. I’d make an exception for the spellings of ‘duffelbag’ and ‘dufflebag’ because of the low search volume each one produces, but ‘duffel bag’ produces enough results that it is best not to try and mix misspellings in.”
It is difficult to write good, user-friendly copy if you keep changing your spellings. This inconsistency can look unprofessional to your users. An exception can be made for low-volume keyword phrases, but ultimately these produce such few searches that they are pretty far down the list of important terms to optimize for. You’d be better off spending your time going after more important, higher traffic phrases.
If appropriate include plural word forms, but do not group plural and singular forms of the same phrase into a single page unless they can naturally be worked into the writing.
P.S. Yeah, I know I misspelled misspellings. The quote is courtesy of Mr. Stoney deGeyter and his fabulous Keyword Research Guide.