Theory and Practice

Yeah... focus on developing Nobility and Scouting as primary skills, and make sure that the number of Chaos magic skills doesn't exceed the number of Nobility or Scouting skills: as I wrote in my previous post, to get a Guildmaster with expert magic (6 skills), you need Nobility and Scouting to be at least 7 (or even 6?) skills each; then the class won't change. ;)

In my opinion, even on a large map, it's not worth it. It's one thing if you find a staff or scroll with a powerful offensive spell, but in other cases, it takes a very long time to develop him. Moreover, if you have Scouting, you want to make a fast-moving unit, and sometimes even a Thief can be useful, and if you have a Noble, why not upgrade his main skill to level 5? +50% to growth is a significant advantage. But chasing after such a rare chance of upgrading isn't very wise, and who's stopping you from moving a Tactician, who will be extremely useful on large maps, to the rearguard.
And with skills developed to levels 6-7, the Guildmaster will be neither fish nor fowl, like a single-shot rifle with only one bullet, and you still need to find armor-piercing ammunition