We used to have a purple reef lobster in a 90. Over the time we had him, he lived with clownfish, green chromis, pajama cardinal, tang, sixline wrasse, and a handful of others.
The only major disaster we had was with firefish - I highly don't recommend them or any sand dwellers/overly shy that spend their time in the caves. We didn't actually witness the massacre, but we did see half of a still breathing firefish floating in the tank one night that had clearly been chopped in half by something. The lobster was the only possible known creature in the tank capable of this, so you do the math. The other two firefish had mysteriously disappeared at separate times only a few weeks earlier, so I don't rule their deaths out either... We never had any issues that we noticed with him and the snail/hermit crab population or with any cucumbers or starfish.
I would agree that if you're hoping to see the lobster on a regular basis, you are going to be greatly disappointed. The only time we saw ours was when he came out to snag food. Luckily we knew where his cave of choice was and it was in the front where we could crouch down and see under the rockwork. Other than that, you'd never know we even had one.
They are cool critters, you just have to be smart about tankmates and the reality of how often you're actually going to see them. HTH!
