Live brine is fine as long as it is still fairly small. The older/larger the brine the less nutritious it becomes. They are quite easy to hatch in small batch's at home, just takes a bit of patience. Be sure that any larger uneaten ones are syphoned out, if any and use a good phytoplankton to keep their food value high.
Decapsulated means the outer egg casing (chorion) has been chemically removed (also eliminating possible pathogens) and allows the shrimp to hatch without expending most of it's energy in the process. This saves most of the high food value for the intended fish and makes them easier for the fish to digest. It also means you do not have egg casings building up and allowing for poor water quality.