Will mods for Legendary Heroes and Fallen Enchantress be compatible with Sorcerer Kings?
In 99% of the cases no.
Will it be possible to use graphic assets and other assets from mods for LH and FE in mods for SK?
As far as I can tell yes! There may be some differences which I haven't really looked at. For example I'm sure you can use units from FE:LH but in that game you are expected to add your own weapon, whereas in this game the weapon comes attached to the model. I haven't looked at what this means yet, but there may be some problems there.
Will SK be as mod-able as LH and FE (I guess yes, since it is the same engine)?
As far as I can tell, yes.
Can you mod your race e.g. the race you are playing in addition to customizing your sovereign.
I believe you can, but this may have to be handled outside the game rather than inside the game.