Hi,
I'm just going to comment on your specific examples.
I am trying to get a grasp on the correct use of nouns, whether typed as variable, uncountable, or mass. Some usages seem permissable, whereas others are not.
a wrench is equipment. -- Now, a typical defnition of 'equipment' is one that consists of the things that are used in one's profession, <<< Yes, although I'd prefer "used for a particular purpose" so the use of this uncountable word with a pluralstic meaning makes it awkward at best for the previous example to be acceptable. So, a better sentence would be "a wernch is a piece of equipment". Yes, but it sounds odd because we'd normally say 'A wrench is a tool'. Is my argument plausible?
milk is a liquid -- I think CalifJim said it is preferred over 'milk is liquid', while both are acceptable. Yes, 'a' is preferable. But in a more scientific context, you might say 'Milk is liquid'.
Last night's crowd was 400 people. We had a large attendance/large attendance -- way above our expectations. -- I think both are acceptable depending on how you perceive the level of attendance -- physically or abstractly. Make sense? 'We had large attendance' sounds very unnatural. You could say 'The attendance was great'.
John's shouting and climbing his uncle's back last night at a family gathering was bad behavior/a bad behavior. -- Here, I think 'behavior' is a variable noun, which I learned can be used countably if used as an example or instance of that. Eventhough both are acceptable to me, since the instance is clearly shown as the case of John shouting and climbing his uncle's back last night at a family gathering, it seem the use of 'a bad behavior' is better. It sounds wrong. Say 'bad behaviour'.
So whether categorized as variable, uncountable, and possibly mass, if the detail is clearly laid out in the writing, using a specific form of the forms will constitue a better form of writing. Not necessarily.
Best wishes, Clive