[BMW Rider]

There is no real reason not to add the live rock right away with the sand, except budget of course. The live rock will need to cure before livestock can be added, so the sooner you start the sooner its ready. The liverock will also do the job of turning your sandbed "live" as it cures. If you add it later you are really cycling your tank twice.

As for using RO water, it is the best choice for a reef system. The hardness of the water has no value for a reef tank, all the neccasary elements are in the salt mix. Thus no other additives are required except as they are naturally depleted in the system. This, as muck stated, is why some use calcium reactors to replace the calcium used up in the tank. Other elements such as magnesium are maintained by supplements and by doing waterchanges. The main concern with not using RO water is introducing unwanted nutrients and other elements into the tank causing algae blooms and other troubles.

And on that topic, you should try to get a skimmer right awy, it will help a great deal to remove the nutrients in the system as it cycles and the LR cures, thus minimizing the inevitable algae blooms. If budget is an issue again, look at some of the designs for building a countercurrent airstone skimmer out of common PVC plumbing parts. They work well, are realativly inexpensive, and would be suitable for your size tank. They can be used without a sump too.