should fairies and spellcasters share a rivalry?

I don't want built-in conflict. I want to decide for my self who dislikes who in my own stories.

I would be fine, however, with settings that players can choose that determine how different types of sims feel about other types of sims. Or even better, a CAS option that lets you choose how a specific sim feels about other types of sims.

Conflict is fine. It just needs to be customizable so players can use it for their own story and world ideas.