Cured rock from an establish tank can be used right away . When transporting the Rock , wrap it up in newspaper to prevent die off.. Most people think that water is the key , in fact water is just a vessel to carry nutrients . The best system is an open loop system where water comes in and completely out all day long..
I just set up a few tanks last week and there ready for sps's already just caus I bought live rock from a 2 year old system that was being dosed.
As far as adding a few rocks I don't see a problem to an existing system , add in one at a time.. And wait a few weeks b4 adding the others, go slow If the rock not cured..