check this http://www.mauifin.com/fin_info.php?id=11
Slalom fins are up to 38, race models up to 52, but they work fine in Slalom.
I would recommend the Race 40 or 42 for the 7,4 in 20 knots. Like the 38, too, which is good for downwindblasting on this sailsize. I do not know the Tectonics and as well not the pricing for Deboichet Slalom fins.