12 gauge is most likely overkill, 10 gauge is certainly overkill, but there will certainly be no harm (except perhaps to your pocketbook) if you choose to go with either of those. I suppose that depending on your speakers and how you connect them, really heavy gauge wire might make it difficult to physically make connections (for instance, if the wire is so thick it won't feed through the holes in your binding posts). 14 gauge will work fine for runs up to 30 feet, and I can't see you running into any issues with the physical size of it.
Connectors are a matter of personal preference. You probably won't be able to get a better signal path than you can with bare wire tightly twisted and screwed down, but if you install banana or spade lugs correctly you probably won't degrade the signal path much if at all.
Sub wiring (assuming an active subwoofer) will generally consist of RCA-terminated 75 ohm coax. This is commonly called 'subwoofer cable' or 'digital audio cable'. A yellow composite video cable will do in a pinch, too.