You can extend the range of you sail by tuning it and adjusting fin size. 38 cm will do fine for an unpowered 7,0 sail or a well povered 7,8. For your 5,7 and 6,4 I would recommend a 32~33 cm fin. It will do fine for the 6,4 when well powered and you can use the 38 in unpovered conditions. I would not go below 32 cm with the 5,7 because the FU has quite a wide tail. Type of fin depends on you practice: slalom, freeride, long distance, B&J...