How many reference letters to submit?
Definitely, having more recommendation letters can prove the popularity of applicant among his/her colleagues in workplace, but just a  negative (or even less positive) recommendation letter can destroy a possible recruitment, as it can strongly affect the impression and judgment of search committees.

A negative (or not-positive) recommendation letter can attract the attention of review committee to weak points (even if they are imaginary induced by the recommendation letter).

It is not bad that a recommendation letter emphasizes on teaching or research only. This is somehow the reason that several recommendation letters are needed (by people who knows the applicant from different perspectives).

Just keep in mind that a search committee always try to foresee any drawback in prospective recruitment. If a recommendation letter states negative points, then, the search committee must consider this risk (for bad recruitment), as there is an evidence for that in the application under review.

NOTE that recommendation letters normally should assist search committee to discover what they have NOT already found in resume. A tiny negative point has more effect on search committee rather than many recommendation letters stating an evident point, they already knew from resume.