I don't know that you have to feed the rock, but go ahead and submerge it in saltwater anyway as planned.
Suggestions, get some eggcrate and cut it 2 inches short of the sides of the aquarium and place that in first, this will protect the bottom of the glass.
Add the sand when it comes in, you will be moving the rocks anyway to aquascape so it doesn't matter how much sand gets on them, just blow it off with a powerhead or turkey baster.
The live rock should be enough to start your cycle but if it isn't a cocktail shrimp is enough to do the trick.
You might even want to fill the tank near full just to test for any leaks.
Get a level out and make sure the tank is as close to perfect as you can. Corner to corner is the most important.
Good luck and keep us posted.
Thomas
