There's another reason the Ares 1 doesn't have fins, which doesn't seem to be discussed in the OP, namely: big rockets have really thick boundary layers. The boundary layer is the layer of air that a vehicle (rocket, airplane, ship, etc) "pulls" along with it as it moves through a fluid, due to the friction between the skin of the vehicle and the fluid. Basically, you don't get free laminar flow right at the skin of the vehicle. If you want to use a control surface it has to be big enough to stick far enough out into the fluid to deflect enough air (or water) to be effective.

Boundary layer thickness doesn't scale linearly with size, which is why small scale model rockets can make effective use of fins, whereas the full-sized vehicle cannot.

Combine this with the fact that, as the OP discusses, a rocket is a really unstable system and fins don't really become effective until you get a high enough speed, and that explains why the Ares 1 needs a gimballed attitude control system in the first place.

Interestingly, really early rockets, like the V-2, had control surfaces that were directly beneath the rocket nozzle, in the exhaust stream. They were, effectively, gimballing the exhaust stream by deflecting it instead of pointing the nozzle. It needed them in part because at altitude the atmosphere was thin enough that the fins weren't effective. You can see them in the picture of the V2 in the Air and Space museum:

http://airandspace.si.edu/webimages/collections/full/A196003...

The little red things close to the rocket nozzle are control surfaces.

Rumor has it that the Saturn V only had fins because Werner von Braun decided that a rocket without fins just didn't look right, and ordered the engineers to add them, even though they were so small with respect to the size of the rocket stack that they were very solidly inside the boundary layer and had no effect whatsoever.

You can see them clearly here: https://www.flickr.com/photos/nasacommons/4858567248/