You don't have to be quite as concerned about "overcrowding" rock that has been cured. I would add it all at once to a new tank.
With "fresh" rock that still has dying things clinging to it, things can get funky if you pack it in too tightly for the surface area to be exposed to good circulation during the curing process. You should see what the bottom rocks looked like after my first attempts! YUK! (Cramming too much rock in a 40-gallon Rubbermaid with too little circulation)
When you get "cured" rock ordered online, there will be some dieoff, of course- but you don't have pieces of rotting stuff that get pinned and trapped the way you do with fresh rock.
