Some 6 string banjos from 1800's have 5 "full length" strings, with the 6th being shorter, often fretless. Tuning?? I agree with difficulties with tone of heavy strings. If your instrument has a guitar neck,(sounds likely)then you could try "Nashville tuning" aka "Angel guitar" stringing with "high octave" half of a 12 string set. Less string tension and more banjo sound with some interesting chord inversions,tuned standard or open.