how do you select a center channel to match when the company only makes monitor speakers?

I would try to see if you can buy a single Harbeth.  Maybe a smaller model within the same line...... It's unlikely another brand will be voiced similarly. ....

I wouldn't bother, just get the best sounding centre you can afford, as it is the dominate speaker in a surround sound setup, doesn't matter if it's a different make. And use your AV amp's many manual setup procedures to dial it and the others in.      

Cheers George