In general, anything that starts with a 14-tooth cog is probably a thread-on freewheel not a cassette. Sheldon explains the difference, but you seem a little uncertain what you have, so I'd recommend reading
this. Freewheels with sub-14-tooth top cogs are pretty unusual, and the ones that exist don't index with Shimano shifters as far as I'm aware, so in your case, a "special hub" would be the only way to get a smaller cog, because you'd need a cassette freehub rather than a hub that takes a thread-on freewheel.
With a cassette, you can customise the gearing by drilling out the rivets holding the sprockets together and then replacing individual sprockets as you see fit, but replacement sprockets for old-fashioned freewheels aren't really available anymore. Also, almost all modern cassette freehubs are designed for an 11-tooth small rear sprocket, so Sheldon's information regarding what works with current cassette hubs is a little out-of-date.