I presume if I decide to go with T6, the C4/C5 would be the recommended center?
Its best to match the series of a manufacturer mains and center channel ...so yes.
Audition audition audition is my advice to you before purchasing. Hearing is personal and subjective and what sounds good to me or others may sound bad too you.
What I would do is to take a pad of paper with you, write down the make and
model of each speaker you audition and what you liked and disliked about the
speakers you were auditioning. Was the bass tight and deep or was it boomy
and loose sounding? Were the mids life like or were they hollow or just too
pronounced? Was the treble irritating and harsh or were they dark and not
revealing or were they smooththat made you want to listen for more? How was
the imaging?
Bring music with you that you are very familiar with and know quite well. To
make it easier to audition HT speaker systems, listen to the main speakers in 2
channel mode with music. Music is much harder to reproduce accurately then a movie soundtrack so if the speakers do well with music, then they will do well with HT. Speakers that do HT well may not do well with music. When auditioning the center channel of the same brand and series as the main speakers, pick a difficult source like an announcer that mumbles alot. If you can understand what the mumbling announcer is saying, then you have a good center channel.
Keep track of what amp or receiver is powering the speakers you're auditioning. Try to get a receiver/amp that closest resembles what you have or want to get. It just reduces another variable when audtioning speakers.
One thing to keep note off. When auditioning speakers, make sure the volume
levels are matched between the diiferent speaker pairs because the louder
speaker pair will always sound better. Listen to levels that you think you
would listen to most of the time because thats how you are going to be using
them most of the time.