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Can you use a spare quick link from some other brands chains for any chain (provided it's the same speed)? I've never tried that and wondered in the past if that would work.
Ignoring things like trying to use a Shimano part with a Campagnolo of course!
It depends on the chain and on the particular quick-link. There are differences between chain and link brands of the
width of the inner link, such that KMC actually sells two different widths of 7-8-speed connecting links, to be used on different derailer chains meant for 7-8sp use.
As far as the pin diameter and the nominal 1/2" pitch, I have measured differences in the
roller-to-roller distance surrounding various brands of quick-links.
This is usually not an issue, but when for example I am connecting a Campagnolo 11s chain using an aftermarket link, I use only Shimano's more expensive 11s link because it shows measurably better installed pitch tolerances than KMC's or SRAM's.
So for this Izumi "Click Index" chain, I would install the narrowest connecting link that will actually install without use of much force. The popular SRAM 8s connecting link is too narrow for all but a few Shimano (IG) 7-8s chains, but might work on the Izumi chain depending on the exact width of the inner link.
KMC's "Missing Link II" was the former name of their narrower (7.1mm vs 7.3mm) 8s connecting link. They now simply state the width on the packaging, so no more "II" designation. most SRAM and Shimano IG chains used the 7.1mm width, and Shimano HG used the 7.3mm width. Note that some early-1990's SRAM 7-8s chains were offered in both of these "nominal" widths, which supposedly reflect the
over-pin measured width, but which is not actually a measure of the critical inner-link width.
Note that
9s inner-link widths became more standardized between brands, but then 10s-width chains came along, again having differing inner-link widths between rivals Shimano and SRAM (using the wrong brand of link can make for difficult installation and severe binding of the connected link hinges, or can allow the link to too-easily separate).
Repeating history, yet-newer
11s chains seem to have again become more standardized in terms of inner-link widths, but chain makers have become more vocal about only using their own brand of link on their chains.