I have a blue lobster (Dr. Zoidberg) in with a XL feeder goldfish (goldie) and some snails. They get along OK, as long as the lobster is reasonably well fed. When he gets hungry, he'll poke his head out of his little log that he lives in, claws raised and open, in hunting mode. If the fish swims in front of him he'll nip at it a little, and every once in a while the goldfish has some nips on his fins. These nips have been becoming fewer and farther between as the goldfish (the einstein of the animal kingdom) slowly learns to keep his distance. I suspect if more fish were in the tank, with less room to escape the lobster's hunting, or if the goldfish were smaller, he would be food by now...
On a side note, I put in some ghost shrimp today, and three of the five have disappeared. I saw the goldfish swallow one, whole, as soon as it was dropped in, but I suspect the other two fell victim to the lobster (they were bought as food). It was interesting watching him chase them around...
The lobster is easy to take care of, just dont keep it with bottom-dwelling fish. Good luck, hope this helps!