Hi Alex,
I agree with everything Carlo and Matthew said.
As you say, Li is very light and so the scattering should be weak compared to Fe. It will also die out much more quickly with "k" than the Fe (or Se) scattering will. So, if you have enough k-range, simply starting your fit at higher k than normal (perhaps 4 or 5 Ang^-1) or increasing the k-weight used in the Fourier transform (perhaps to 3 or 4) would de-emphasize the Se-Li scattering to a level that it was safe(r) to ignore.
FWIW, I would imagine that trying to fit coordination number or sigma^2 for Se-Li at percent-level concentrations would not work very well. If you did indeed get values that were clearly telling you that there was definitely Se-Li scattering contributing, I would wonder if there was something else going on (say, from another ligand, some multiple scattering, or some other phase).
The formula for the Einstein temperature is a scale factor times "coth(theta/(2t)) / (r_mass * theta)" where t is the temperature, theta the Einstein temperature, and r_mass the reduced mass of the atoms in the path. See
for details. This will not include S0^2 -- they are conceptually totally different.
As Carlo said, the sigma^2 in the EXAFS equation does not distinguish between static and thermal disorder. But if you have temperature-dependent data, modeling the sigma2 values as a static offset + a term that depended on temperature with an Einstein model would be a fine way to go.
Hope that helps!