While I haven't overwintered Calibrachoa, I have successfully overwintered many tender perennials. I keep them in my basement under fluorescent lights. I use inexpensive shop lights with one warm and one cool bulb. As for watering, I would water sparingly making sure they don't dry out completely. Not sure what I would do about pruning, but I might be tempted to try rooting cuttings so I would have extra plants next year. It's worth a try.