All the cinnamon roll recipes that I've seen used all purpose or bread flour. I'm surprised that they would add the cornstarch since you are working with a yeast risen dough. Yes adding the instant pudding would act much the same as adding cornstarch. If you are happy with the results that's all that matters.
If you ever want a substitute for cake flour remove 2 tablespoons per cup of all purpose flour and add 2 tablespoons of corn starch per cup.