Though they normally are priced per pair, they come single boxed. I don't know for sure, but I can't think of a reason on earth why they shouldn't be willing to sell singly.
I use a CC-470 for my center. It's drivers match the 20's very well and it sounds excellent but I imagine that using a 20 as a center would create an even more seamless soundstage. I believe that, for additional money, they can have it magnetically shielded.