Inflexion of foreign loanwords

How should foreign loanwords that inflect in the source language (like for number and gender) be inflected in English? (I suspect this is a slightly more generalised version of Plurals of loanwords of obviously foreign form, with similarly vague answers - ie, there is no definite rule - but I'd appreciate confirmation).

In particular, upon reading a text that says her petit body I cannot help but think that this is incorrect English despite son petit corps being the correct inflexion of the equivalent French adjective to match the masculine noun.

Perhaps that is because calling a lady "petite" is rather commonplace in English (the equivalent French being la dame est petite), whereas using the French adjective to modify a reference to her body is less so. To me, if one must describe her body as small using a French loanword, then her petite body seems more natural in English.

Are there any authorities that opine on this at all?

What's the plural of 'octopus'?

In general, French words should have French inflections and English words should have English inflections.

A loanword starts as a foreign word and, if used in English, it should be inflected according to that language's rules. Once it has 'gone native' and become an English word, it should be inflected using English rules - unless the foreign inflection has also transitioned.

Due to the influence of Old Norse and the simplification of English following the Norman Conquest, English lacks traditional gendered nouns and adjective inflections. Gender inflections of foreign loanwords tend to die more quickly than other foreign inflections like pluralisation. There is still a gendered distinction between blond/blonde, latino/latina, fiancé/fiancée, brunet/brunette, nouveau/nouvelle, prima/primo, etc., one of each of those pairs is far more common and the other is relegated to idiomatic usage, when it is still used at all.

However, your example isn't about loan words

Petit is a French word; petette is English, albeit of French origin. Neither is a loanword. Now, I'm not saying you shouldn't dump a French word into the middle of your English prose, but you should have a reason for doing so, and that will inform how you inflect it.