I kept an arrow crab with a pair of reidi for a few months and did not have a problem myself. The arrow crab also did not kill any of the bristleworms, but did like eating the seahorse food.
Others keeping arrow crabs with seahorses have reported problems and the larger the crab gets the more problamatic things become. I think I got away with it because my arrow crab was rather small.
Bristleworms are not as bad as most believe, they are actually quite good as a clean up crew. If you use a feeding dish and limit the amount of food they have access to, then you will be able to control there population better then by biological methods IME.
I did have a fireworm in my system once and removed my rock and did a hypersaline dip for 10 mintutes with water measuring 1.040. This caused the bristleworms to flee the rock, by the hundreds and I removed the 4 foot fireworm. I did have hair algae problems that followed. The hyper dip will not damage the bacteria on the rock.
HTH