I doubt Silvers will get a new slot for free.
Pretty much a given for Subs to get at least 3, so they can run one of each faction.
If you sub for one month, you will also get more bank and inventory space, 500 Zen and a couple other stuff I can't remember atm.
If you level any toon to max level while you are subbed you will also get a free ship? Not sure if that has ever changed.
You will however gain free boff slots and starship slots for leveling up while gold.