This may be an oddly specific question, but I've run across comments online that suggest the following usages found in Pliny the Elder's Natural History would not be valid in the Latin of Cicero:
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Ad reliqua transeamus animalia et primum terrestria.
Maximum est elephans proximumque humanis sensibus,... -
Velocissimum omnium animalium, non solum marinorum, est delphinus, ocior volucre, acrior telo, ac nisi multum infra rostrum os illi foret medio paene in ventre, nullus piscium celeritatem eius evaderet.
I would interpret "maximum" and "velocissimum" either as adjectival modifiers of a noun "animal" that is omitted by ellipsis, or as nominalizations of the adjectives (with the same sense as the construction with "animal"). I'm not sure how much difference there really is between these interpretations. In any case, the use of the neuter doesn't feel incredibly exceptional or rule-breaking to me: while a simple predicate adjective in Latin is expected to agree in gender, these cases clearly have something a bit different going on (in fact, I think it's arguable whether the nouns "elephans" and "delphinus" in these sentences are the subject or the predicate, although maybe someone can answer that for me).
Here's a comment I read that I find confusing:
The genders and numbers in that first sentence are a bit confusing, too. Pliny starts out with maximum and proximum in the neuter, because he’s thinking of animal, but Cicero would have written maximus and proximus (or maxumus and proxumus) in the masculine because of elephans. The same happens later with delphinus est celerrimum omnium animalium. This [is] not the most classical Latin. Then he changes into the plural and uses illis because he’s thinking of elephants as the species as a whole.
(Barbanaira, Oct 2020, Language Learners’ Library: General Chat, bolding and word in brackets added by me)
Is it true that Ciceronian or "model" classical Latin does not allow this kind of usage, and would instead call for masculine forms in these kinds of sentences? If there are examples or counterexamples of similar sentences from Cicero, that would be appreciated.
Other sources I've found:
Allen and Greenough gives the following comment on the dolphin example: "A superlative in the predicate rarely takes the gender of a partitive genitive by which it is limited" (286 b)
Kühner's Latin Grammar says "A word in the predicate, especially a superlative, connected with a partitive Gen., takes the gender of the subject, and not of the Gen., as: Indus est omnium fluminum maximus (not maximum, after the gender of flumen)." (page 222)
Another online discussion thread about this kind of usage (in French):