When not to use an adjective and not 'of'

I note that in your example "four species of butterfly" is equivalent to "four lizard species". Lizards are of course know for their conciseness.

n

"Fit as a butchers dog" is an expression used to explain that an individual is very well physically. Even though the original translation was to explain that it meant well fed.
It has nothing to do with how attractive a person is. This expression is very old and calling someone "fit" for being beautiful is a fairly new addition to English slang.

I prefer your version. Yes, the form 'snake species' is used nowadays, even though 'snake' is not technically an adjective; but I try to avoid such a usage if there is an alternative.
I would say 'species of butterfly'. 'Butterfly' is being used as a general term.

I like the final version. My background is science and engineering. It's been a lifelong quest to achieve the most concise grammar which is also interesting. Hence the final version gets my vote.
I note that 'species' is both singular and plural. therefore both “species of butterfly” & “species of butterflies” should be correct.

Sorry if they got away with it, but the editor turned your beautiful sentence into a catalog entry. Re dangerous men, there is a clear difference between the two phraseologies. If all six of the men are considered dangerous, this would be the right way to phrase a circumstance where they became viewed as such independently, especially if they are not connected to each other. "In those six provinces, police are investigating prolific burglars responsible for at least 100 burglaries each. All six of the burglars are considered dangerous." This is a better way to phrase it than "all six burglars are considered dangerous," because it gives life to the idea that a burglar is not necessarily dangerous, but in these cases all six of them were viewed that way, perhaps emphasizing a further point, such as the possibility that a prolific series of non-violent crimes could turn the criminals into violent offenders. Saying only that all six are considered dangerous loses that sense and is kind of a dry way of describing it. Maybe in a police report, but not in a public statement in say a newspaper.

Likewise I would prefer saying species of butterflies to an audience of children, while in a technical journal, if butterfly is meant as a scientific family or genus or whatever, I'd want to keep it singular. As in, 13 member of the (Family that is named for) butterfly.

Addendum: For the "all six men" option, this would be a better description if the six men are connected somehow. "In the prison break, six men escaped. All six men are considered dangerous." This phrasing indicates what can not be assumed, all of these men are considered dangerous. It points out that in a group of inmates, they are not all necessarily dangerous.

If they escaped from the crash of a prison bus and a cement truck, the best description would be "police are on the lookout for six hardened criminals."