You can chelate yourself with ALA alone and get well. But adding DMSA to it, can keep things more stable and speed things up a bit apparently.

It shouldn't be used after new exposure to mercury and/or when blood levels are still high. Or if you have redistributed mercury previously and repoisoned yourself. Those things need to be reduced first (with DMSA or other methods or just waiting it out).

Selenium is not a chelator according to Andy, so does not reduce mercury. I tried it when I was mercury toxic and did find I had reactions which may have been some detox, but I dont think it's a strong enough agent to significantly reduce mercury. I think it can displace some of it though by competing for binding sites. Plus it helps raise glutathione levels, which may help indirectly with a certain amount of detoxing.

I am really not sure about selenium, it's an interesting one and caused both good and bad reactions in me. Those really mercury toxic may find they have to start with low dose selenium becuase of reactions.

Andy discusses ALA and selenium and much more on this link:
http://home.earthlink.net/~moriam/ANDY_INDEX.html

Just scroll down till you get what you're looking for.