If you can get the letter co-signed by a prof, that'd be nicer. As a supplement to a professor's letter, I don't think it'll be a problem. Just don't send over 6 letters to a school unless you get permission - even at 6, you risk irritating them by forcing the adcom to read a lot of material.
For what it's worth, I used a letter by a grad student who knew me well, co-signed by a prof. Currently, I've got 2 acceptances, 1 waitlist + defer, 1 top ten waitlist.
I prefer to think of letters as serving a purpose. You don't want to send a school ONLY letters from science professors who can only talk about how good your work was for one class. You want to send a mix. Consider getting letters from people who've watched you grow up from grubby-handed freshman to grizzled senior, maybe one from your job, or perhaps from a humanities class you loved.
Getting into medical school isn't just about showing the admissions committee how awesome your GPA is. While that's incredibly, and sometimes, the most important thing, you stand a better chance if you can also show the committee a face behind the name. You're not Applicant X, 3.62, 29O. You're Firstname Lastname, a guy/girl who loves music/running/panda bears/slam poetry/rainy days/serving up meals at the shelter/short essays/long essays/etc. If you can get letters that can help show off your personality and other intangibles that have no place on a secondary application, you'll be better off... unless your personality is horrible.