For the skimmer, you dont want to wait too long. Assuming you are going to re-cycle the tank and start slow, you can compensate with water changes and low bioload for a few months. Bev and others run their setups without skimmer at all, but are religious with maintenance.

And for the rock, yes you can add it in slowly. Don't add too much live rock at one time to a tank that is already cycles, or the die-off might spike your tank. You can add as much dry rock at a time as you like.

What I did with my first tank when I couldn't afford all the LR right away was buy 1/2 my LR and 1/2 cheap base rock.The base rock gave some surface area for bacteria.And added more LR later.