I have my bourkes, linnie, and green cheek on the fine, too. However, all of those birds were eating other pellets before I started them on Harrison's, and the old pellets and the Harrison's pellets are exactly the same size, so that made the switch a breeze.
Daisy, like Jenny I would recommend that you go with the high potency at first, especially since your cockatiel has not eaten pellets previously, so may not jump right in, and also because he is molting and perhaps even further stressed.
If he is plucking, is there any way you can have a vet check him for a possible physical condition? I know some people do not have any avian vets near enough for that, but if you do, that would also be a good idea.
My vet told me NOT to get high potency for Ashlynn, even though she is a grey and a baby. Why? Because she was eating like a hog and had gained a significant amount of weight in less than a month.
She is definitely MY bird.