"Old Timey" style Bluegrass will only require a couple of area mics and one centrally located 'solo' mic.This is not only true live, but in the studio too.Typical old school bluegrass players like to stand roughly in a circle and get their insrumental blend through hearing it as well as feeding off of each others licks.This will require a room that has quality sonics to capture this style and still have some separation for mix.Theres not a lot of mixing though, as the players generally do this on the fly.
Modern Bluegrass is different.This is as much like your standard country/rock sessions as anything.The basic tracks usually go down much as a country tune, with bass,guitars and a guide vocal and drums if there are any.And these days there might very well be.You wouldnt chose to use heavily colored preamps on any of these acoustic instruments.Clean,crisp, and clear is the order here.As for mic selection, good small D condensers on the guitars, a real quick responding LD on the banjo,mandolins sound great with an LD in front and a SD over the shoulder,if theres a drum kit, a minimal setup is all thats needed.A couple of overheads,a snare mic and a kick.The kick doesnt need to be close miced as the boom is desireable.Goboes in the room for some separation might help, but this kind of basic tracks will have "Good Bleed" if properly miced.
By the way....Bluegrass is one word.