I use your stack size combinator extensively throughout my Pyonadon base for loading trains. Would it be possible to add fluids to their own combinator? I know there isn't a specified stack size for fluids so either having 1 or a value that can be set in the settings would be greatly helpful.

You can turn the alphabet combinator into a fluid signal only combinator (on newly placed alphabet combinators) by commenting out lines 185-192 in control.lua. Commetning out a line is done by adding "--" in front of it, like you can see in line 183.