Add to Mitch's summary: it is always safe to connect a speaker combination whose impedance is more that what the amplifier is rated for.

E.g.
Amp spec 8 ohms, speaker can be 8 or more e.g. 16
Amp spec 4 or 8, speaker can be 4 or more e.g. 8, 16

"Safe" in the sense that a transistor amplifier won't overheat because a higher impedance speaker means less current flows.

Valve ("tube" in the USA) amplifiers are a bit different as they tend to limit current safely, so you are less likely to get damage if you connect a speaker of too low an impedance. With any mismatch, however, you'll not get the amplifier's maximum possible power.

Joe: "if two 16-ohm speakers are wired in parallel, that halves the resistance and makes it 8 ohms. And I suppose they would make 32 ohms if wired in a series."
Absolutely correct.