Frank,I play the guitar without any picks,I cant grow my finger nails because of concertina playing.
I am used to fingerpicking guitar,with thumb index ,middle ring,the natural thing for me to want to do is to use index for third, middle for second, ring for first[up picking] thumb for fourth downpicking[as I do on guitar].
however I understand,that would alter the emphasis, particularly in bluegrass eg if a thumb is meant to pick down and it is replaced by an index picking up.
a lot of the bluegrass rolls are coming easily[Ironically I dont like bluegrass much,but prefer old time].any suggestions would be welcome.