I agree he should explore the possibilities of adding different sub-classes to himself and his closest comrades but watching the episode again, he didn't actually give anything to Rudy. It was actually a status granted by the "guild". I think that is a strong distinction because there might be limits to what guilds can "grant" as a reward to members. As for contract quests with EXP rewards, I guess its possible but don't forget he used some pretty rare items to achieve that magic. He probably does not have an unlimited supply of ink and paper. One of the many reasons he wants to keep the contract a secret more thank likely.
Also last thought on the different sub-classes....since they are all level 90 (level cap) could they take advantage of other skills without the ability to level up? Maybe the first step is finding away around the level cap.
Edit: Words