It all depends on your preference, and the conditions under which you ride. I'm a bit shorter than you, but I have a 17.5 in frame for my hybrid, a 15.5 in frame for my city commuter, and a 14 in frame for my winter commuter. I find that the smaller the bike, the more nimble it is (think about BMX bikes).

For longer rides with few or no stops, I prefer a longer, taller frame. For riding on black ice, I prefer a very small frame (shorter distance to fall )

If you are comfortable on your bike, you made the right choice. You might find the small MTB limiting if you decide to do a century, however.

As for the slicks, if you aren't doing too much dirt or gravel riding, you'll probably find that they are much faster and quieter than knobbies. There are lots of cheap slick tires out there for 26 in wheels.

It wouldn't hurt to try a slightly taller frame and see if you prefer it - just trade with one of your friends who has one and ride it around campus. That will answer your question better than all the BF speculation in the world.