Actually the only thing you have wrong is loading the flange up with Silicone RTV. Your Kohler cast iron sink is a good one and will probably outlast several disposers. You may want the option of removing the flange and using a nice new shiny one off one of your new disposers at sometime. Despite what the Silicone RTV users claim it does have very strong adhesive properties making removal a bear of a job.
The cast iron sink is much thicker than a stainless sink and the flange will fit. However, if you are using the Insinkerator style flange with the snap ring I would recommend a second person to push down from the top while securing the ring. It can be a lot of fun to do alone there is very little room left.