Learn French: Vous devriez utiliser un pseudonyme, mais sans insulter autrui. - You should use a pseudonym, but without insulting others.

Autrui is an indefinite pronoun meaning "others" or "other people" in a general, abstract sense. It is often used in formal, moral, or legal contexts. You could use les autres here, but autrui elevates the tone and emphasizes "other people in society" as a broad concept.

Does autrui change form if we are talking about a specific group of others?

No, autrui is completely invariable. It never takes an -s and only ever refers to people in general. If you wanted to say "without insulting the others" (meaning a specific group of people you were just talking about), you would have to use les autres.

The English translation says "without insulting", so why is it sans insulter and not sans insultant?

In French, all prepositions (except en) must be followed by the infinitive form of a verb, not the present participle. So, "without doing" translates to sans faire, and "without insulting" is always sans insulter. English speakers often trip up on this because of the English "-ing" ending!

Why is devriez used here rather than the present tense devez?