We would always advise pairing the center channel speaker of the same series with your other speakers in the system. 

For example:

Monitor SE - SE 2000C

Premier - Premier 600C

Founder - Founder 90C

Persona - Persona C

We do also offer smaller center channel speakers for the Premier and Founder series, Premier 500C and Founder 70LCR, should there be any constraints on the space that these can be placed in.