Congratulations on your recent win. I'm jealous.
I currently have the 6T's and use a 5C for center. I have not had any issues or noticed any difference between the speakers. Aperion takes pains to timber match their speakers so there should be very little to no noticeable difference between a 6B and 5C. I do have a different brand for surrounds, and I can notice a difference with those in video games when I swing around and move voices to different speakers, but I never notice it with the front 3. Also, depending on what order you get the speakers, you always have the chance to return them within 30 days for free, or upgrade within that year.