Wow, NINE emerald crabs?!?! Are ya sure about that? I only have four in my 240. I think that yours might starve pretty quickly since you have so many. Plus, they will fight with each other when they get bigger.
That's a really nice zoa rock you've got there -- it looks really big. Be sure to post pics of it when it opens up!
Oh sea cucumbers... In my opinion, they shouldn't be kept in reef tanks. Some people like them, they do a good job, but like you said, they bury themselves in the sand. That means, IF one dies in your tank (like what happened to me), you are not gonna find it. Which means that it will ROT bigtime (they are REALLY bad when they die). I think it was a dead cucumber that caused my infamous nitrate spike where it got up to 150, and I could never really get it back down to pre-cucumber death levels because that sucker just slowly rotted in the substrate and constantly fouled the water.