If your tank is cycled, the BB can take a big hit if changing over to brackish quickly. That said I am not sure how long it would take to change over the tank gradually to become cycled or partially cycled.
If your tank is not cycled yet and you have recently started the tank, then you can change it over more quickly. A lagoon will have differing mixtures of saltier water and fresher water mixing in from a FW source and from the tide going out and in. Gradually would be over a few days maybe if not trying to save BB.
Marine sand, and Reef Crystals or other basic saltwater fish type salt is good, you do not need specialized reef growing salt. You can add in other lagoon / estuary plants. Those are beautiful fish.