If I were you I would put all 3 clowns in the tank at the same time, making sure that two of them are definitely smaller than the first and hope for only one female and 2 juviniles among them. Any male/neuter can change to female, but once female there's no going back and there would be defnite dominance issues. The ones that have not changed to male/female are neuter. Optimal would be to only add one more. They're best in pairs (unlike chromis who do better in odd numbers 3...5...).
As I'm sure you're already aware the Tang is an ich magnet. I would try to minimize his stress during transition as much as possible (catch/acclimate/move at night) so that he's less likely to fall ill.
The stuff in your current tank is definitely 'cured'. That just means that everything that's going to die in mass on that rock has already died and broken down and the rock is no longer a risk to cause an ammonia spike.
After looking at pics of LR from LR.com it looks like my impression of the LR from the LFS was niave
I was hoping you'd see their stuff. I'd never seen anything quite like it before locally. They take it straight out of the ocean (after planting it there themselves) so it is as fresh as fresh gets and teaming with life and since it is aquacultured they're not destroying any part of nature by removing it.
Just think twice about those damsels before putting any rock in. Most people who have them are sorry they do and are looking to get them out!