Simon,
Dont' change anything. Better stick with iSonic 86 for 6.5, 5.6 and 5.0 sails with your 70 kg IMHO.
Either iSonic 96 or 101 will be too large for your light weight and your 2 smaller sails combo (more indicated for a 6.0-7.5 sail quiver range; check out
http://www.star-board.com/forum/showthread.php?t=2478).
Cheers !
JM