Seeing as how I'm the country music guy of META, I guess I'm best qualified to answer your question. Steel guitar in Nashville has no rivals and Paul Franklin would probably be considered the best in town, at least the most versitile. With Paul's rack of outboard gear, driving a pair of speakers I always mic his steel with a pair of Sennheiser 421's. Dan Dougmore uses a single combo amp which sounds best micing it with a Shure 57. I've tried lots of other mics but always go back to these two mics.
For Mandolin, I usually use a Royer 121 at the f hole and a Neumann Km 184 pointing down at the strings, where the musician is strumming, from about a foot or so above and out.
I'm tracking with Stuart Duncan beginning Thursday and I'll get back to you about the strings after that. As for micing mostly I use an AKG C12 on fiddles although I'm really into DPA's 4041-ta, large diaphram tube mic as well. I always individually mic fiddles.