Everyone has a different chemical make-up and what works for one may or may not work for another...it's a process of trying to see what works best for you. I am sure if you are working closely with your psychiatrists that the two of you will come up with something that will work well for you.
As to prednisone not causing mania...this medication is a stimulant and will increase the chances of going over if your borderline. I know....they say it can't, but I am here to say that it can and will. My suggestion would be to let your psychiatrist know of any and all medication changes done from other sources so that he or she can adjust and compensate.
One of the biggest hurdles facing psychiatrist's is the lack of information from patients when their doing other medication changes not related to the illness. It's usually discounted as not having any effect, whereas most do have an effect and some combo's can actually kill you.