The black one is probably Haptosquilla glyptocerus. It won't get much bigger and isn't a threat to anything you're likely to keep in a reef or fish-only tank. It will hide in a little cavity and just yank mysis and other meaty bits from the water column. It might take out tiny snails, but anything that you're likely to keep for "cleanup duty" (astraea, turbo, cerith snails; blue- or scarlet-leg hermits) will be "safe enough".
The brown one is one I can't recognize definitively from the pics that you provide, although I suspect Gonodactylellus viridis from the color, and that it shares it's range with H. glyptocerus. Do you know where it came from? Also, if you could get pics of the tail (straight down from the back), and the inside of the smashers (look for colored spots), they would help greatly with identification. In any case, it's not any of the "tear the tank apart" species (peacock, etc).
Dan
