I am thinking of adding a new coral crab to my 3 month tank!
1) Will my damsels / Clownfish attack the crab?
2) I already have a Fire Shrimp & a Ugly Brown unknown crab. Is it okie to have living side by side?
3) Are all crabs harmless to reef/coral?
Not at all! The vast majority of crabs are NOT reef safe. I would try and get an ID for the one you have already before adding any more small fish, shrimp or coral to the tank.