Not everyone wants to invest a lot of $ in fins and hope to find a single fin that covers a wide range of sails.
I am one of those. While I have the iS 111, I did buy one fin (Tectonics Tomcat 42) that works really well with all three of my 8.4, 7.6 & 6.6 Maui Sails TR race sails. I have found that the 42 hasn't been too big in 20-22 knot winds with my 6.6. I am free sailing and not doing downwind slalom and weigh 77kg.
If you are doing downwind racing then go (2 cm) a smaller than what I suggest below.
Tomcat 38, Goldwing 38 or Goldwing 36. Each could each make a good "one fin" quiver. The smallest Tomcat available is the 38.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.