Well that's good! I kept getting that image, or the image of chewing on an un-inflated balloon when you said rubbery, but firm is good. While you may have cooked them a little too long, I don't think it was a disasterous amount of time.
I'd say that the skim milk had a part in the matter also, as well as the reduced yield.
Did you eat the little nubs (pimples) after brining or air drying, or just out of the press? After brining and drying, the rind can become quite hard and pretty rubbery I would suspect also.