Let me start out with a single bluegrass singer. I would think that if there is only one person who sings and plays, he or she would probably choose a guitar which can be used to accompany them with interludes of picking the melody. I guess that any type bluegrass instrument could be used alone.
What type of instruments do I think would be used. If it is a quartet, than a guitar, a banjo, a mandolin, and a bass. A violin, a dobro may also be used.
By all means, any brass or woodwind instruments should never be used. Neither should a piano nor an organ be used. It seems that most all bluegrass instruments have strings and are easily carried around (to whoever's porch they are sitting on, picking and grinning).
This is Saturday night and I am watching the bluegrass music shows on the RFDTV channel. They also have country-western music shows on, too. And yes, I like those also, but bluegrass is my favorite. Now, you all have a great day, you hear?