If I remember correctly, arrow crabs can either be good, or they can eat anything not nailed down, depending on the specimen -
Stenorhynchus seticornis (Herbst 1788), the Caribbean Arrow Crab. Not to be trusted with small to medium fish tank-mates (may spear with rostrum, otherwise consume). Safe with hardy native corals and anemones, larger fishes.
Can also go after just about anything else smaller then a medium fish, from what I read.
As for the bristleworms, there are a few options..
You can build a trap for them, which I've had very little success with (piece of pvc
capped at both ends, drill holes appropriate for them to get in, put food in it and submerge, turn out the tank lights and wait).
You can buy a fish that'd eat them, but that is generally a bad idea - that said I have an arabian pseudochromi that lives in my live rock wash tank (to flush out bad critters)...he keeps them in check. In the right environment, this type of solution makes sense, but not for your display.
I've had pretty good success using anti-aitasia liquid, the reef safe stuff (chem-marin Stop Aiptasia).. Take a small amount in the syrenge and shoot it in the hole of the rock that they come out of. They will come out rather quickly (except for the larger ones, which is what I was hunting) to find a more pallatable home. Grab em with tongs and discard.
That all said... bristleworms are a good thing for your tank as they are a good way of taking care of leftover food or detritus in the substrate. I can understand however if they are large or very prolific the need/want to remove them.