Regarding giving it a fire spell, yes, maybe I went overboard with that. At least because the fire sign will most likely be applied after the creature has run out of magic charges. It's a shame that in Heroes 5, you can't give a creature an elemental auto-attack; that would be ideal...
If you use these fire golems in conjunction with some fire mages, then well, maybe it will work, why not. Although, with the Fire Shield, you could consider removing this, just for the sake of balance.