Here is a reply from Polk (who, btw, are always very quick to reply):
A very balanced system would be using the RC65i in all of the positions, a single RC65i for center, would give ideal component matching across the front three speakers. Another approach is to use the TC65i as the center, since this can be bought as single speaker, owners like not having an "un-used" speaker if they buy a pair of the RC65i. But, if this doesn't bother you, then the RC65i will work fine.
Regards, Ken, Polk Audio