I own a lionfish (everyone agrees that he is a russell) and he is fully grown at 12'...he does not bother any of my sea urchins (they will eat any left over food...i know this because during the time my lionfish had his jaw broken i would leave food in the tank for him, but the sea urchins would eat it >.<), snails, or any of the countless bristle worms in my tank
while he was hanging out at the lfs as i was changing to a larger tank the crabs did disappear (however he was being housed with a grouper so not 100% if it was him or the grouper) personally i don't like crabs they seem to be little trouble makers, to be able to get a large enough crab not to fit in a lionfishs mouth, might end up being more trouble than what it is worth
the other problem with lionfish is that they have great eye sight and aim...whenever he gets hungry and the tank lid is open he will attack anyone with water (my friend kept blaming me for splashing her with water >.<) now my foxface does the same thing...he LOVES attention and the camera and will swim up to anyone who approaches the tank (i believe it is more for the love of food than for company ^^)
lionfish are known to get lackjaw, that is why it is VERY important to give the lionfish a mixed diet (scallops, black tiger shrimps, mysis...ect) and make sure it is mixed with vitamins (i personally use IM needle to inject vitamins into the food before feeding) they are very easy to train to be feed using tongs or your hands
