Bristleworms are not bad.
Most tanks have them, but a lot of people don't notice them because they generally come out at night and a lot of people don't stare at their tanks after lights out.
Iv'e got bristleworms in my tank, they've never hurt anything. They only eat things that are dead or dying. They also eat extra food that hits the bottom of the tank, so consider them a part of your clean up crew.
I think that more of your problem with your clams could have been related to the fact that you had 4 clams in your nano tank. I don't have clams or anything, but 4 of them in a nano seems like they would have been really crowded and most likely not getting enough food.
If your tank is over run with bristleworms than you've got a bigger problem going on.