Thanks for building this, it is helping me a lot!

Inverting fluid signals does not work due to this line:
https://github.com/modo-lv/factorio-mod-stack-combinator/blob/cbe138c391e0268f9af6db68431d4ccda4d0d1ce/scripts/staco/staco.lua#L77

Looks like entry.signal.type can have the value "fluid":
https://lua-api.factorio.com/latest/Concepts.html#SignalID

Since the stack size of fluids is always 1 maybe this is intentional? Honestly I don't have a usecase for this, and I'm not sure what one would be, but I dug into the issue because I thought I was going crazy when some of my signals weren't being inverted as expected.

There is now (since v1.4.0 ) a map setting that lets you control how StaCos process non-item signals.