I like to keep them around for the most part. They eat detritus and add to the cleanup crew. Once they get to big feed the tank and get them with tweezers.
I've got some of the bright red little monsters that have hit me a couple times (ouch) when cleaning, usually hiding under rocks. I have a six line and a melanurus but I've not seen them get one yet. Back in the early 90's I got a bunch of live rock from Hawaii that had some of the variety that get huge. By the time that tank was really mature I was seeing some come out at night that were in the 10-12"+ range. Nasty things that would stand up like a cobra and if bothered would spray some sort of black liquid. Killed some of them but it was my coral banded shrimp that killed most of them. On a couple occasions I saw clash of the titan battles at night. Only wish I had of had video back then.
Hard to say... you just never know what aggression you will get from fish or inverts. Last time I had a CBS (large) pretty sure I also had at least cleaners (always get a pair) and he never harmed them or any fish. But he stayed on the opposite end of the tank and everyone stayed away from him. Almost always out and very aggressively standoffish though.