I think it depends on what stop you're at. If it's Central or Harvard Sq. I would focus on single-speed conversions. Like it or not, lugged, steel frames are the new black and college kids don't want to mess with pesky gears. Stock up on Raleigh Grand Prixs. I think including 3-speeds (w/fenders) is not a bad idea. Put a Tourist in your window, but when they can't afford it sell them a Fuji S-10.

Edit: Given your size, you might consider becoming a coffee shop that happens to sell bikes. I didn't know how bad I wanted a bike until that Super Course showed up at my local coffee shop/LBS.
http://www.crccoffeebar.blogspot.com/