All 5 speakers in my setup are identical. Makes for a nice, seemless sounding system.
Not sure about using 2 bookshelf speakers, but one should work fine. The problem with putting two right next to each other is you will likely get a more pronounced cancellation effect than you would from the typical CC.
Is your center pulled all the way out to the edge of, if not slightly further out from, whatever it is sitting on? The number one problem I usually see with centers, is less than optimum positioning. Have you played around with the positioning?