Are ion channels/resistancies in Rall's model voltage-independent or not?
If you are interested in passive diffusion and not just membrane potential generation in a dendritic compartment, then you need to look at the cable equation:

$$\frac{1}{r_l}\frac{\partial^2 V}{\partial X^2} = c_m\frac{\partial V}{\partial T}+\frac{V}{r_m}$$

Here, every compartment is connected in series with resistances $r_l$.

We define the length constant as 
$$ \lambda = \sqrt{\frac{r_m}{r_l}}  $$

and the time constant as
$$ \tau = r_mc_m$$

Then the equation can be re-written as:

$$\lambda^2\frac{\partial^2 V}{\partial X^2} = \tau\frac{\partial V}{\partial T}+V$$

The reason $G_e, G_i$ are not mentioned when talking about diffusion along the dendrite is that they are only involved in the generation of voltage $V$ in the compartment.