I have a Bristleworm (Fire) in my 35g. Never knew it was in there until one day when it emerged to get some food that had made it to the bottom. It is the largest I have ever seen, at 1 and 1/2 feet in overall length. I found out the hard way that it was a fireworm, my hand didnt even work for two days!
I find it to be a great scavenger, and it has never harmed anything in the tank, except me.
Aarow crabs devour the smaller ones if you dont want it in there, My arrow crab see's the big one and runs for cover, smart crab!