For grounded emitter CE designs, though, you can just as well pick 25mV instead of 26mV as your starting point and then just imagine that gain is:
5. |Av| = 40⋅Rc⋅Iq, Iq = quiescent current, Rc = collector R
The "40" comes from 1 ⁄ 25mV.
So the little re is added to the existing emitter resistor, if any, before dividing into Rc. If there is external emitter resistance still present at AC, though, you add that to the little re.
I didn't mention this yet, but equation (1) actually applies in ALL three dimensions. So that energy is the most probable for each of the three degrees of freedom. In fact, all degrees of freedom in equilibrium states, will have that. In complex molecules in gases, which have many different kinds of vibrating motions within the molecule as well as molecular motion within the macro-environment, will store ∝ k⋅T energy (actually, ½kT) in each mode, not limited to just the 3 dimensions anymore. But in this case you need only consider the degree of freedom related to the gradient of the voltage potential, which is usually quite linear (1D) in electronic devices.