soldering speaker wire to speaker

You shouldn't be holding it there long enough to cause it to heat up significantly, anyways ...  If you do, you need to work on your soldering skills ...

Of course, I wouldn't solder the wire to the terminals, anyways.

There is really no reason to use anything other than copper wire, and no real reason to use anything larger that 16 AWG.  There is no meaningful measurable difference in signal integrity between silver, copper, silver-plated copper, oxygen-free copper, or solid gold wire.  Most reported differences "heard" by people are completely subjective and at least partly due to the listener desiring to hear a difference (AKA the placebo effect.)  All genuine double-blind tests that I have ever seen or read about support the fact that people cannot reliably hear any real difference in speaker wires - or between interconnect cables, for that matter.

If it makes you feel better, if you believe you hear a difference, or if you simply like the looks of it, more power to you.  But my recommendation is to spend your money on better speakers (no matter what you're currently using) before spending it on expensive or esoteric wire.

Correct.  16AWG.  Typical losses would be less than 0.3db with 1200 watts into 1 ohm.  Only an issue if you are a serious competitor.