this blog:
http://blog.cakewalk.com/windows-8-a-benchmark-for-music-production-applications/ indicates that X1 works well on win8 and MC6 is a stripped down version of X1, so it would not be a stretch to say that MC6 should work on Win8, however, there's nothing definitive from cake stating that explicitly that I can find.
for stringed instruments such as fiddle, banjo, mandolin, you can find those in the TTS-1 GM2 soundsets.
beyond that, those types of instruments are difficult to sample for good emulations, so they are more expensive than things that come in the Sound Center packs. fiddle, or violin, are found in kontakt, sampletank and others.
banjo and mandolin might be found in them, too, but I don't use MIDI to for those, so I'm not sure. it's more likely to find them in a package like the realstrat where it emulates playing as well.
another option you might want to consider is Band in a Box. they're probably about to release the newest version, so I'd wait for that to be released before buying. but they have "real instruments" which are basically audio loops played by real players or by MIDI and are configured to the key/chords you state and are output as audio files which can be dragged or imported into MC.
www.pgmusic.com