Would it be too much to be able to level multiple classes for our retainers, similar to how we can level every class?

I think it would add some additional depth to the retainer system. Make it so you can only switch their class once a week or something.

The only real downside (outside of coding it) would be that people might buy less additional retainers, but I think most people who have additional retainers value the extra storage and ventures per hour. (I have 3 total retainers, and they're all pretty full and busy.)