Both of my boys go by a "C" name, and the girl's name we have picked out is a C name too. I feel a little weird doing a totally different letter for a third boy, although the C's weren't intentional, just the names we liked.
I'm kind of overall worried about this baby being left out since my boys are about 20 months apart and this baby will be 3.5 years younger than DS2. I worry that DS1 and DS2 will continue doing the same activities, sharing friends, etc and this baby (especially if it's a boy) will get left out a lot.