The rule is you can add cured liverock to a tank with livestock.
The problem is knowing if its cured, semi cured or uncured.
You can buy rock that is advertised as cured but is in fact only partially cured or becomes partially uncured by being left out of water for too long or being kept in incorrect conditions.
If you add rock that is not fully cured to an established tank, you run the risk of an ammonia spike, which will kill the inhabitants.
I bought some fully cured LR from a repuatable place, that basically died in transit overnight as it was very cold, even though it was still wet. It took over 4 weeks to cure again before I could add livestock.
To sum up, just be sure its fully cured before adding to an already stocked tank. Best to add all your LR before adding any livestock, rather than adding in bits and pieces.
Hope this makes sense,