It looks like the 121 will be a good call given your carve and smaller sail.
The custom fin idea is also a good call, they do make a difference. Same deal with the 121 if it comes with the stock drake fin(s), you will get better range and control with a quality fin. I use a Tetonics Tomcat 42 on my iS 111 and love it. The Drake fins that came with the board were not good.
Good luck with the 121.
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.