I generally recommend against mixing L/R and Center speakers from different manufacturers. It doesn't mean that you can't do it. What happens is that you will hear a noticeable change in sonic signature for HT movies as sounds pan from left to right. An example would be a motorcycle driving across the screen. The change in sound will be very apparent when the sound start on Left speaker and then pans through the Center and finally to the Right.
Like akg_ca said, for HT, the center speaker is the absolutely most important speaker because 90% of the sound and dialogue comes from that speaker. It depends on your priorities.
For surrounds, it is my opinion that they do not have to match exactly, but they should be somewhat close in signature. I am running B&W front speakers with some older Focal surrounds and I don't have a problem.