They certainly don't decrease any levels of anything in the water. Don't know where you heard that. The only thing you would have a problem with, is if you had too many corals, they could start a chemical warfare with eachother. Also, if you have tons of coral, the demand of calcium will be pretty large, so you would need to start dosing that. But that's a another story..
Coral in general don't add to the bioload of the tank, such as fish and inverts. Feather dusters are inverts, therefore they DO add to the bioload of the tank.. but very little from what i've seen. Nothing compared to fish. And yes, coral and inverts do better with low nitrates.