Re, Gaon; yes he did learn nistar young. So did the ramchal; they were the exception, and were able to do so without being damaged by it. Most people who go into kabalah early or before they’re ready have serious problems. This is an example of the exception proving the rule.
Why do you think thus takanah was made? It was because people were going wonky over kabalah that they learned before being ready. This jncluded shabsai tzvi.