This is a very good question. In general, you can help yourself work through any confusion by trying to express the idea more precisely or in a slightly different way. I'll use each of your examples to explain this a bit further.
EX1: All lizards have a head width, but that isn't what you measured. You measured the different head widths of a group of lizards, or "the head widths of some lizards." I would at least add the "some", but perhaps it would be better to include the actual number.
EX2: You are talking about the growth rate of all birds, not, say, the growth rates of different species of birds.
EX3: Each of two populations has a mean height, therefore there are two mean heights. You are saying that the mean heights of these two populations are different.
EX4: Dogs have noses is correct, but this is a bit inconsistent. For example, dogs have a sixth sense that lets them know when someone is coming, and they have a nose for trouble. People have minds, but they have a combative streak. So, when you are talking about a shared characteristic of all dogs, you use the singular, but when you are talking about something that each dog has, you use the plural.
tardy
noun
an occasion when you are late for a class, or a record that a teacher keeps of this
MacMillan
The plural is tardies.
See also Wiktionary:
tardy (plural tardies)
(US) A piece of paper given to students who are late to class.
The teacher gave her a tardy because she did not come into the classroom until after the bell.
Before turning to the dictionary I had posted the following:
As to saying that tardy is an adjective, well it is also used as a noun. Someone can have one absence and one tardy this week. Or someone could have two or more tardies.
We can indeed convert adjectives to nouns by using only the first part of an "adjective-noun" combination. Consider smiley face. That became smiley. Oh, my gosh this girl likes to use smileys in her texts.
If a word is used as a noun, it's a noun.
Best Answer
"Minima" is the Latin style pluralization of minimum. In spoken American English, I think minimums is much more common (and my spellcheck recognizes minimums, but not minima), but either is acceptable.
To read more about Latin plurals, check out this link.