Dude there is no telling what you'll get on live rock. 99% of it is pretty harmless or even good for your tank bristle worms are one of the best scavengers going and they are harmeless 99% of the time. Copepods and amphipods are the basis of the tanks food chain. I've gotten stars, urchins, tunicates, sponges ,"nothing know to man smells worse than a dead sponge", crabs, hermits, atapsia, algae etc..... and have seen more than that so your guess is as good as mine. The beauty of using mostly base rock to start with is that you have a whole lot more control over what is in there.
hth
Max
hth
Max