Thanks. Glad I could help.
Yes, since the rock has die off (from shipping) it will start your cycle. That's a great way to do it. Keep an eye on your water and do water changes whenever your ammonia gets near 1ppm. Above that and the ammonia will start killing the bacteria you're trying to raise.
Once ammonia starts really falling feed the tank a bit of fish food every few days to provide food for the ammonia fixing bacteria.
Yes, scrub the rock off in a tub of saltwater. Most of the things you scrub off can come back. I use a new toothbrush for this. By scrubbing off the decaying matter you help keep your ammonia from spiking too high.