Bristle Worms are actually a hotly debated topic by some.
I have many in my tank and generally feel they are beneficial.
They cleanup the leftovers, I see them sneaking out at feeding time for scraps and especially after dark.
If they get especially large they can become a nuisance, but be careful removing them, they can pack a bee like sting.
I have never been able to attribute any untimely deaths directly to bristle worms myself.
I don't think you can buy them, but if you have bought liverock they are probably in your tank already.
Be careful of the Chocolate Chip Stars (I think this is what you are referring to with the red spikes). I have read they are not entirely reef safe. Same goes for Green Brittle Stars.
They have been known to take small fish when sleeping.
YMMV, proceed with caution and do your homework first.
I learned that lesson this time.