You're probably going to want to go with the Polk CSi3 or CSi5 to be timbre matched.
.. corrected it for you.
Those were the RT centers from the same generation as RTi6. The current year CSi A4 and -6, while not completely timbre matched, will also work better than either CS1 or CS10.
edit: to answer the usual follow up question whether the larger center is "worth it".
I started with CSi3 and RTi6 years ago when they first came out, and replaced the center with CSi5 a year and a half or so ago. While the difference is less than the size alone would imply, I am not looking to go back. The potential of CSi5 and similar when properly placed and amplified, is HUGE.