I definitely would NOT use another brand of center speaker. At best the overall sound from the front would be mediocre or it may even ruin the sound from the Harbeths. Take a look at this article,
http://www.soundandvision.com/content/matching-front-and-surround-speakers
And a mismatch between front L&R and center is more degrading than front and rear.
An absolute perfect matching center speaker would be a single Harbeth Super Plus, or any other Harbeth should be good. As suggested, try to find a single Harbeth. If this is not possible, here are some other options using 6 speakers.
3 front (L/C/R) and 3 rear (L/C/R)
3 front (L/C/R), 2 sides, and 1 rear
2 front (phantom center), 2 sides, and 2 rear
Phantom center can work well, but only when listening positions are on the center axis or fairly close. Extreme off axis listening positions from the center will not be good.