The bioload produced from the snails, shrimp, and coral are negligable, so I wouldn't worry about putting those in at once. I put my two clowns in at the same time so that they were together from the beginning, and I believe most people do it this way, so I would say that's fine too, especially if they are small. The only thing I may worry about would be the live rock. Is it cured? If so, I would think you should be fine since it is a small amount. If it is not cured, or if there is any die off, there can be an issue, especially in a smaller tank.