Then, it is likely because the row in appearance.2da has no entries for the various body models, that is likely what is causing the crash.
So if you want to use that appearance row as a PC disguise you will need to copy the model and text entries (likely in the underwear A slot) all the way down the line for all the different body models B to N. Then you shouldn't have a crash when donning different armor/robe items while having that appearance.
I just looked at TSL's appearance.2da and you will need to, on the Sith_Assassin_01 row, edit in N_SithAssn
which is in the modela
cell you will need to add that entry to the modelb, modelc, modeld, modele, modelf, etc. all the way to the modeln cell.
Save that appearance.2da to your override then try the disguise again.