Maximum number of recommendation letters for assistant professor position
When reading references committee members evaluate them based on different evaluation criteria. Some place a lot of weight on the worst opinion voiced. In that respect, having more references than needed is a risky prospect. Another point to note is that you’re never 100% sure that all your referees write the most enthusiastic letter possible. For example, they have a student on the job market at the same time. 

In my experience, additional letters carry little benefit and potentially expose you to a lot of risk.

While this varies widely by discipline (and by department I’d imagine), reference letters (in math/CS at least, but again YMMV) carry more weight when they’re from big shots. In that respect, a letter from a junior collaborator is not going to have as much impact, especially since this is a close collaborator who will most likely say only good things about you.