Gonk is correct (as usual). Signal propagation in a wire is at the local speed of light, or about 70% speed of light in vacuum. Moving one speaker an inch further away from your listening position will create more delay than adding 100 feet of wire, so don't worry about it.

I do recommend keeping wires similar length, but only because it helps match series impedance on both sides. 20 vs. 10 feet of speaker cable might add a tenth of an Ohm or so, which can theoretically change a speaker's voicing a tiny bit. Is it audible? Almost certainly not, but if you can make them similar, do so. So I agree with Ritz above about matching pairs of speakers.

This assumes you're using identical speakers in all five (or seven) locations. If not, don't worry about it at all; differences between speakers are greater than you'll notice with extra cable length. My mains have matched lengths but the rest are all different.

12 gauge is fine for just about anything. I use spade lugs, they're cheap and work well. If you go with bare wires, solder the ends solid to keep a lone strand from fraying and shorting out your amplifier... seen it happen...