Even if you can go narrower, possibly all the way to 23mm or less, you have to decide if it's desirable on a commuter.

Narrower tires don't necessarily offer lower rolling resistance, though they are lighter. The flip side is that commuters are usually riding poorer roads in heavier traffic, so the chances of dropping a wheel into a pothole, sewer grate, or expansion seam on a concrete road are much greater. Also, it's starting to get dark earlier which makes things that much worse.

I ride 25mm tubulars on the road, but use 26x1.9 slicks at 60/50psi for the commuter. There's a marginal difference in how they roll (if any), but these monsters are immune to all the road hazards urban riding offers. They also have a fringe benefit of higher air volume at lower pressure. This allows me to make it all the way with a small leak. Most of the flats I deal with are found in the morning, which spares me doing on the road repairs. Of course the on the road repairs are always in the rain.