I really, REALLY wouldn't reccommend selenium for a mercury-toxicity patient from anything BUT whole-food sources, like the paranut (macadamia nut).
I have 11 amalgams, and took selenium for six weeks at 200 mcg last spring. I developed poor photosensitivity (delayed reaction of the lenses in the eye to react to different light sources), motion sickness when riding in cars, and found out a little late as to why..
Selenium binds with mercury, yes, but the body recognizes it as an essential nutrient, and the selenium-bound mercury is transported to nurons throughout the body, including brain tissue. The selenium-bound mercury is capable of being transported across the blood-brain barrier.