Yeah Murray, you're right about the "C" bass. I generally use a "Masterclass" 56 or 58 on the bass of my o-18. They're an unusual kind of low tension string made in Britain. The 58 plays like a 53 but can be tuned down to C with little "flutter and wow".
Here's a trick that I've been doing for thirty years and it REALLY works (especially on 12 string).
Cut a tiny V shaped groove in your capo where the bass E meets the rubber. You'll never have to retune 'cause the E went sharp when you capo. If I'm using heavier guages for the E AND A (which I often do) I have a capo with two small "Vs" cut into it.