Regular townies turning into spellcasters?

Thanks for this thread. I was able to take precautions in my game to mostly avoid any existing Sims from being chosen by the game as Spellcasters. What I did was, I went to the Gallery and downloaded a bunch of Spellcasters and stuck them in Newcrest, then evicted them and went to the household manager and un-favorited them. I also tried to choose a variety of ages. I did this before going into any households to send Sims to the Realm. When I finally got there, I recognized some of the Sims I had downloaded.

Unfortunately, it's not failsafe, as one of my own Sims that I haven't played yet but who lives in Sulani, a pirate, turned up there casting spells. But as it turns out, he looks really suave as a Spellcaster so I might just keep that as his little secret. Maybe it will help him catch that Mermaid he's been chasing. >:)