I have a custom template save that I use to start every game save. In that save are copies of all my favorite Sims I've played over the years as not-in-world townies. All the premades have been modified in some way, whether just adding cold/hot weather outfits when Seasons came out, or more in-depth physical modifications. I never actually 'played' any of these Sims, simply used the 'manage households' feature. All premades are still living in their original houses. All of these Sims, premades and my own townies are in my favorites section in household management.
I have two spellcasters in two separate saves using this template and neither one of them has met any of these Sims. I was extremely surprised when they went to the Realm and none of the premades or my townies were there designated as spellcasters. Every single spellcaster, other than the Sages and Glimmerbrook residents were brand new game generated not-in-world townies.
So, my conclusion from this is that your Sim doesn't really need to 'meet' or interact with premades or townies for them not to be chosen for spellcasters, they just need to be interacted with by you somehow for premades or be your own previously played Sims from other saves.
However, I have three Sims in this custom template that are always vendors in every single world, every single save. One of them is the premade Zoe Patel, the other two are my own played Sims from previous saves I put in the template. These three do show up in the Realm as vendor ghosts in Casters Alley. That's it though, they are not spellcasters. That's just been my experience so far, perhaps I've just been extremely lucky. Just thought I'd share in case it's not just luck, but by design and others can try the same strategy to avoid premades autonomously becoming spellcasters.
I do have the problem with situational outfits for my Sages though, it's frustrating and I hope they fix that soon. I can ask my Sages to see their every day outfit, but they just change back to the situational outfit the next time my Sim visits the Realm. Curiously, although the Untamed Sage, Morgyn, has also been to festivals and such, they have never been stuck with an outfit. I've noticed though that they never change into any kind of situational outfit there anyway. This Sim seems to have been coded differently than the other two Sages. At least in my game.
Even though I don't have the problem of premades and/or my townies becoming spellcasters (at least so far), or use mods, thank you to those who posted links on how to remove spellcaster traits for those that do. <3
