A good question in linguistics would be general: something about the nature of language. People do ask specific questions about specific languages, but in a good question it's clear how to relate the specific question to something broader. Since you're talking about something made up, it wouldn't be possible for the reader / answerer to relate the specific question to the big picture. I cannot figure out what the underlying general question might be. In addition to answers being completely arbitrary, I find the question literally incomprehensible, in that I cannot identify the presuppositions or re-phrase the question to be something linguistic in nature.
The revision refers to a world with French spirit summoners, but there are no spirits or summoners of actual spirits, so you would need to clearly identify the hypothetical presuppositions. Questions that ask if something "makes sense" are often bad because they fail to explicitly identify the rational basis behind a prediction, and it would be best to reduce this to something like "Does outcome X follow from the meaning of the term Q?", in which case you've directed our attention to a possible issue in semantics.
Here is an analogous question about English words: "Would it make sense in American English, given the meaning of "cow" and "pig", if cows were never called 'pigs'? Or would you expect cows to always be called 'pigs'?" I'm not trying to mock your question, I'm trying to strip away the confusing presuppositions and reduce the question to its logical bare bones. Basically, it follows from the conventional meaning of "cow" and "pig" that the things called pigs are not cows, and vice versa, so as long as people use words according to convention, then you predict that people won't call cows 'pigs'. More sophisticated semantic questions could probe the relationship between "penguin" and "bird".
Here is a paraphrase of your question which IMO is better (but maybe totally misses your intent), though I don't claim that it is good. "In a literary context that has witches (or whatever) who summon spirits, would it follow from the meaning of the term "Familiar Spirit" and the assumed nature of such spirits that they are never called "Familiar Spirits"; or would you expect them to always be called "Familiar Spirits". My paraphrase switched from French to English (was there something about the French term that was crucial?), and I got rid of the difference between "summoned spirit" and "familiar spirit" (is that a technical distinction? If so, it needs explanation). Unfortunately, this paraphrase is like the "calling cows 'pig'" question.
If the question can only be answered by someone who is intimately familiar with fantasy writing practices in French and is about stylistic norms, I'd say that the question is totally off-topic.
Based on the "unicorn" information, it seems that the kind of question that you're asking about is the probability of a particular term being created to refer to some thing, especially when the term is historically or synchronically composed of identifiable parts. An example would be "Since the marionberry was tested in Marion County by George Waldo (who worked in Benton County), from the Chehalem and Olallie blackberries, is it most likely that the berry would be called 'marionberry', rather than 'Georgeberry', 'Waldoberry', 'Chehelallieberry'?" or "Logan Blackberry"? Certain possibilities could be ruled out on linguistic grounds, like "Bbery" (pronunciation) and it turns out for the same reason "Chehelallieberry" (for reasons having to do with stress conflicts and h), but the remainder are equally likely from a linguistic POV.
While linguists are generally aware of the source of the component parts "uni-" and "-corn" and might, given some elementary knowledge of medieval culture be able to see a reason for "unicorn" over "onehorn", that's just not doable in the case of "Familier". The question might be suitable for a site on literary history, because the heart of the question is "what were the real-world circumstances surrounding the historical association of this word with that concept, centuries ago".