Example mod looks good to me but I wouldn't give fay horses to Hakkapeliittas tho. I would like to keep recruitables as mundane as possible.
I was thinking of making also 2 spells which summon hiisis. Possibly nature-death spells. One which summons multitude of small hiisis (goblins) like Burnsaber said, and other one which summons bigger ones, more like jotuns or trolls. Atleast thats what springs to my mind when Im thinking about landmarks like
Hiidenkivi,
Hiidenkirnu and Hiidenkiuas which were considered to be work of hiisi.