I think if he copied the weakness then it would be the weakness that resulted form the actual ability, rather than any physical weakness they have. Like if he borrowed Red's ability to see and hear spirits... the voices would flood in and overwhelm him, give him headaches, whereas if he copied Sophia's ability, his legs wouldn't be paralyzed as that was caused by a car accident he did not personally experience. 

Does he store them or just borrow one at a time? It would be neat to have an arsenal of abilities, albeit weaker, at your disposal :3