The front speakers should be at ear/eye level, particularly the center. It's job is to focus your eyes and ears on the screen so it "fools" the brain into thinking the sound emanates from the screen. Think about how it will be with your eyes looking at the screen but your ears hearing from overhead. You might get away with the rears being in the ceiling although it would not be my choice.
As for that center speaker you mention, it sux, and I rarely come right out and dis any speakers, but I'll make an exception here.
Rethink your speakers in general, particularly that center speaker. It's recommended that at least your front speakers be from the same manufacturer and the same line.