Mine are kept in mid-brackish levels, around 1.010 specific gravity, they eat like pigs and seem very healthy and happy. They have solid lines, and were kept in brackish water at the pet store.
When making a tank brackish, you need to use sea salt, I like Instant Ocean, but any marine salt will do. "Aquarium salt" is for freshwater fish when you want to kill parasites or help with ailments (as was said), so no, you probably didnt make your tank at all brackish by adding a few tablespoons....it takes a LOT more salt then 2 tbsp/ gallon or whatever, its more like half a cup every 5 gallons I think....
Anyways, for a 10 gallon tank, you need one "5 gallon" box of Instant Ocean (its pre-measured), which will bring your 10 gallon to around 1.011 SG - mid-brackish.
As for your BBG's, just go back to the pet store and have them read the specific gravity for you, if its brackish, then add a bit of salt water gradually untill it gets to the same level as the pet store....but from what Ive read and heard, BBG's are almost always brackish, I could be wrong though.
Mine love bloodworms (frozen), and "people" shrimp.....they eat peices of shrimp that are bigger than their heads