Bristleworms are good, what's the worry?
You can battle them all you like but they are a standard plague in seahorse systems. Seahorses produce a lot of waste, there digestive track does not fully digest all of the food they eat often. This leaves mysis for the bristleworms to munch on.
The best way to limit there numbers is to limit there acsess to food. You can use a feeding dish and siphon out any uneaten food, or poop. Limiting there food source is the easiest way to control them.
There are other option such as removing all of your rock and doing a hyper salinity dip. Measure the water to 1.040 and put the rock in a tub with a powerhead. In about 10 minutes the bristleworms will have fled the rock, leaving a pile of eewww in the bottom of the bin. When you dispose of the worms I would seal them in a plastic bag, they tend to have a fragrant smell very quickly IME.
There are various traps you can make by putting food in a mesh bag that some people like to do, and pulling out several worms at a time.
IMO I think you should go with the learn to love them option, or the ways of removal i have suggested.
A wrasse in that size tank is not going to work out longterm. There swim movements are likely to bother the seahorses which often leads to the horses not leaving there hitch, not even to eat. Which is bad. The food competition would require you to feed so much more you would just be making the worm problem worse IMO.
Arrow crabs and CBB shrimp don't belong with seahorses IME. A snap from either on a seahorse opens the door for secondary infection, which is the biggest threat to seahorses developing disease. There is almost always an intial trigger IME.
HTH