6mm is fine.
I use that exact same setup. See http://aquakeeper.com/nano_setup.htm
You don't need a DSB! A small monthly water change will do it more neatly and nicely than a dsb for your tank.
Approximate amt of salt is written on the bags/boxes. get a good brand like Tropic Marin/Instant Ocean/Kent Marine. They cause less problems than cheap brands. And when you mix saltwater, mix it for at least 6 hours with a powerhead or invertebrates and maybe fish will suffer when the slightly corrosive water is added to a tank (the chemicals in the salt have not fully dissolved)
Light? If you have even the most basic corals/anemones, at least 2-3 fluorescents. If just fish, none.