Jackman should follow the Rock's regiment if he wants to be completely bulked out for every role. The Rock has never actually been slim in his entire life, so Jackman would do well to follow someone who is actually slim. Then again, I think the whole world would be sad to see a muscleless Hugh!
Yes, Hugh Jackman may need to slim down for upcoming roles. Many actors and actresses do. Whether he follows The Rock's bodybuilding and fitness tips or not should really be up to him and possibly his trainer. The Rock's tips are fairly extreme and not a good fit for everyone.
Hugh Jackman needs his own program. This is because every individual is different. When someone has enough money, a celebrity for example, they should hire a professional fitness consultant to get them in their prime shape. Although The Rock's tips might be helpful, The Rock isn't a medical doctor to be giving out tips.
Each individual is different and their body can respond differently to certain diets or fitness plans. What works best for The Rock may not work as well or at all for Hugh Jackman. He should consult his trainer and dietitian to see what plans would work best for him considering the demands of his upcoming roles.