According to Cutler, the ALA is what's good for removing mercury from the brain. He recommends using DMSA too, but he doesn't say it's absolutely required (I think I'm remembering correctly. My memory is better since I began chelating, but sometimes I wonder). Does the ALA worsen your reaction or just the DMSA?
Some people use just the ALA for chelating. You could consider trying it and seeing if it works for you. I don't know if using ALA alone with DMPS might cause any issues though.