to add a little to this fin discussion, after few months on is101 '08:
it was difficult to really get used to the thought to use 42 cm fin in 96 litre slalom board, but from my experience in the end it worked relly well in low to medium wind (~12-17,18knots). in higher wind (~15-22,23knots) i used ,with the same sail, 38 mfc slalom fin ,mostly because i couldn't find a way for stock 36 fin to work (frequent spinouts) and based on taty's recommendations. in my experience this fin was a good allround fin, but did not really excel in any conditions. it was ok when the wind got lighter or stronger, sea surface rougher or flatter and occasional spinouts were result of to much bakfoot pressure from my side (as opposed to 36 drake that spinouted in when you could least expect it). it's main advantage was the price that was 2-3 times lower then benchmark deboichet fins(after seeing on mfc web site that they also have custom fins i can only guess that taty actually used custom fins?).
for smaller sail i found a perfect match in select superfast 33 fin (5.5 sail)