Hi Krysse,
I am on 50mg prednisone daily. I started on 60 mg prednisone about 3 months ago. It has helped the muscle weakness in the legs and arms--not so much with the double vision.

I did experience some increase in symptoms when I started the prednisone,but that has subsided.
Prednisone has a lot of nasty side effects, as others have mentioned, and you are probably already aware of b/c of your experience with it. I really limit the amount of salt I eat in foods, as well as sugar. So far, my blood pressure has been fine, I haven't gained any significant amount of weight, and my blood sugar is under control. I think I've been one of the lucky ones with dodging some of the side effects.
I can't really answer your questions about supplements. I'm also on cyclosporine for the MG and that drug interacts with so many others that I just try to stay away from supplements.
My neurologist has also told me to adjust my Mestinon dose depending on how I'm feeling; but I've been given an upper limit and told not to exceed that dose.
I hope your father-in-law sees some positive results from the prednisone.
And he sure does have a special daughter-in-law!

Cate