.....An established tank can keep its nitrogen cycle for over four days without any maintainance. After that the bacteria slowly go to "sleep." After a month, they'll need major rejuvenation....but still better than nothing. You might feed it a little fishfood if you're not going to stock it for a couple of weeks.
LL...Your (cycled?) gravel should have been in conditioned water.
cjz....Crushed coral will tend to raise your pH. Is that what you want?
As for whether it's a good plant substrate, you might get more better answers on one of the plant threads.