Introducing evolution into the Hodgkin-Huxley equation
Although you mention evolution, I think what your question is really asking is in your last paragraph: what happens to conduction within dendrites and axons when geometry changes. There might be some evolutionary implications of this question, but you will also find vast variations in geometry among neurons in any one organism, as well.

You might want to look into expansions of Hodgkin-Huxley into compartment models that allow you to test the influences of geometry, see wikipedia for a list of different models at different scales of complexity.

There is a tool available called NEURON that is good for student or curious non-student experimentation, though you will have to read the documentation and hopefully find some examples that are close to what you want and then tweak them.