I use my iS 111 (2007 model) and a 7.3 cambered sail with a Select Elite S05 (custom series) in 39 cm size; S05 is a older version of new S07.
I think for a 7.3 (and for a 7.6) 39 cm fin is a good choice.
For a 6.7 sail I will probably use a fin in 37 cm size (or maybe 35)... all depends on your needs (i.e. speed over upwind angle) and wind and water conditions.
SL7 is the prepreg (and cheaper) version of Elite S07 fins; till 40 cm size I think both models are rather similar in performances; over 40 cm I think Elite series is definitely better.