This is something I have never done. A lot of people say that fish are at risk when they sleep if you have anything with claws and pincers in the same tank. Also, in order for crabs/lobsters to grow they need to shed their shell every few weeks. They become extremely vulnerable because they are incredibly soft for a period of time so you would need to provide a really good place for it to hide.
I couldn't possibly tell you whether it will be okay or not, personally, I wouldn't mix the two, stick to fish and if you want to keep lobsters, keep them on their own, or with their own