CAN I MIX AND MATCH SPEAKERS?

Yes. Mixing and matching speaker brands and styles is fine. In fact, the whole concept of THX certification is based on the idea of being able to mix and match components from different manufacturers. The THX certification process standardizes all electrical and output parameters, so that all THX Certified speakers and receivers work together.
You can potentially have box speakers for the Front and in-wall or in-ceiling speakers for the Surrounds. However, THX recommends that you keep groups of speakers similar. This means, the Front speakers should be from the same manufacturer and designed to work together. Surround Left and Right speakers should also be identical to each other, as should the Surround Back speakers.