There was a question a little while back on the English SE asking about the "plural form of i.e." (unfortunately, it got closed because the author didn't clarify what they meant).
While I was trying to answer that question, I faced the problem of not being familiar with exactly how "id est" is and has been used in Latin. I know it is the source of the abbreviation "i.e." that is commonly used in English texts today; it also seems to be equivalent in general to the English expression "that is". As I say in my answer post, my understanding is that the Latin expression, like the English one, is invariant in form, although at the time I was only able to find one example of "id est" being used between two plural noun phrases.
To increase my confidence in my answer there, I thought I'd just ask a more general question here: are there any differences in usage to be aware of between Latin "id est" and English "that is"? If there are any differences in the usage of "id est" between different periods of Latin, I would also be interested in learning about them.
I.e. being used to mean "namely" or "to wit" is a very literary construction. Classical Latin was often a written record of oratory or prose written as if it were being spoken. In speech, its equivalent would be something more verbose like "namely...", "to wit...", or (more Latiny) "which is to say..."
Id est in classical Latin is generally going to be a literal reference to a literal neuter-gendered thing that is under discussion.
Phaed. Perii hercle, huic quid primum dicam nescio.
[By Hercules, I'm lost. I don't know how to start talking to her.]
Pal. Em istuc, quod mihi dixti.
[Pffft. Just tell her that thing you just told me.]
Phaed. Quid id est?
[What thing is that?]
Pal. Periisse ut te dicas.
[Tell her you're lost.]
Phaed. Male tibi di faciant.
That's just for emphasis. Most of the time, an "it" or "that thing" is just going to get thrown into the verb and left implicit: it'll show up as est without a subject at all. If you've got more than one neuter-gendered thing under discussion, it's just going to naturally become ea sunt or—since the ea is understood—just sunt.
[Edit: See @Cerebus's answer for examples of a clarifying id est already being used in antiquity.] I.e. came into English through medieval Latin, which used tooooons of abbreviations to save space on valuable sheepskin. It was also highly literary, so that you get constructions like i.e. to obnoxiously clarify a point where you think you've lost your (naturally) less educated reader.
Like you thought, it'll generally be invariant since the id being discussed is "that thing I just said", regardless of how many words or examples were specifically involved. E.s. isn't really a thing, though you could employ it if you want to underline just how better educated (and obnoxious) you are.
In addition to Ily's overview, I'd like to offer a few examples in which the expression is used just as in English:
Ennius, Varia 1 (from this translation of Euhemerus' Sacred History) 140:
inque sepulchro eius est inscriptum antiquis litteris Graecis ΖΑΝ ΚΡΟΝΟΥ id est Latine Iuppiter Saturni.
Cicero, Pro Balbo 10.7 & 24.12:
contra foedus enim, id est contra populi Romani religionem et fidem fecisse dicitur
servos denique, quorum ius, fortuna, condicio infima est, bene de re publica meritos persaepe libertate, id est civitate, publice donari videmus.
As one can see, id est was used by the Romans in cases where it does not refer to a specific neuter substantive (in the first Ciceronian quotation, it refers not to foedus but to the adverbial phrase). In addition, id est was used in cases where a specific substantive could have been referred to, such as ZAN KPONOY in the Ennian quotation. It stands to reason that invariable id could also be used after plural substantive nouns. I would probably never use ea sunt.