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Please post all questions, including technical questions, in the Rubber Side Down section."

That being said...

When they say your tires should be matched... they are refering to the traction quality of the tire.  As an extreme example, you shouldn't have a dirt tire up front with a gran prix tire in back.  So you should be ok with the same model of tire front and back whether it's used or new.

Valve adjust is every 3500 miles.  You're over due by 2.