The disk brake question is primarily one of whether the fork/rear triangle have the mounts for them or not. If yes, then switching to disks would be fairly easy, though you might need a new wheelset as well. If no, then switching to disks would be so expensive as to be really not worth it. I've had a couple of Co-Mos from around that vintage and it would surprise me if it didn't have rear disk mounts. I think that model came with a steel fork, so wouldn't be surprised at all if it has the mounts for the front as well. If it did have a carbon fork, then probably not. There's no particular reason not to use a disk on the rear and rim on the front if only the rear has the mounts.