For an unpowered mixer connecting to powered speakers then normal 'instrument'/guitar cables are fine. I wouldn't use anything that's two flimsy though as these cables will not necessarily be tucked away and you'll probably have longish cable-runs.
For a powered mixer or separate amplifier connected to unpowered speakers then you would need different cables. Many people use 10-amp 2-core mains cable for connecting amps to unpowered speakers.
The reason is that the output from an unpowered mixer - a 'line-level' output - is at about 1 volt. Speaker cables have to deal with many tens of volts - an amp I've just bought puts out about 40 volts onto the speakers.
That's the simple answer, but I think it's enough to answer your questions.