I have never had a salt tank before but I have been researching it for a long time so I am pretty educated on it.I want to make some like rock with cement and want to know if I NEED to put salt in the cement mixture and afer I have it made and cured I will have it in a seperate tank while I grow "stuff" on it.How do I grow stuff on it?Can I take a small piece of rock from the ocean and put it next to it and let it grow stuff on the DIY rock?

correct, just add a piece of live rock, and give it time and patience.