It really is important to be skeptical of any heavy metals test you take. They are not known for being extremely reliable. If you have a lot of the most commonly described heavy metal symptoms and the test comes back negative, I would not automatically assume that heavy metals are not a problem. I've read a number of stories of people whose tests came back negative, but they chelated anyway and felt better.
Actually, I've also read about people who don't seem very sick and have heavy metals tests which are off the scales. This makes me wonder if the tests are always accurate the other way too.
Some people have trouble with DMSA. I didn't have serious candida flare-ups while using it. I'm sure there are people who have trouble with DMPS as well. A lot depends on the individual. The key is to find a chelator that works for you and use it.
I recommend that you steer clear of store-bought chelator blends. The different chelators have their own schedules to follow, so which schedule should you follow with the blends? It's also important to know which ingredient is working for you and which one may be causing you a reaction. It's much better to use chelators and supplements individually according to one of the established protocols.
Right now, I'm using ALA on an hourly schedule during the day. It seems to be working well, though it's too soon to say for sure.