I've done some playing around and figured out a very simple way to add these 16th based real instruments into a 8th notes based tune at a speed of 140bpm (any speed really). Simply go to soloist and chooses bluegrass band from the top and when the options come up choose "all banjos" this will change your song temporarily to a 16th notes base at the speed of 70 and lengthen the chords. Then go back to solo and get the real bluegrass instrument that you want and it plays beautifully.