I have also a problem with this (macOS Catalina 10.15) - some SoundToys plugins would immediately crash Studio One when inserted on a track. But I noticed that only some will crash it (like Sie-Q, LittlePrimalTap, ...) and only the VST versions - the AU versions run just right, didn't experience a crash yet with any of them. So you could try to use AU with SoundToys.