I think the difference between a 25.5” and a 26” would be negligible for drop C. There are plenty of 25.5s out there ready to go that would tune to drop C, arguably, every bit as well as a 26” scale length. I’ve heard a few people talking about that scale length lately, but can’t think of anyone who builds it regularly. However, if you’re gonna get a custom job done anyway, I don’t see a reason not to go with the 26”. I would definitely recommend experimenting with heavier gauge strings before making that investment though. After all, I’ve seen guys tune 25” scale length guitars to drop B, and they seemed to live with it just fine.