Re: String gauge for C tuning?
There's no right or wrong here so I'll take a stab at the question
I like to match the string gauge to the scale length of the guitar. If it's a 25.5" or similar, i think normal 12s would be fine, as I can use 11s in standard, E flat, and D and they don't feel too slinky.
For a 24.75", I'd take one of those "low tuned" specialty sets. GHS makes theirs with the string's cores actually bigger than their normal sets, and the plain strings are 11 gauge but the wound ones are closer to those in a 13 gauge set. I would try that approach.