I think, halflings should be specials for human provincial armies.
But for me the idea of providing different choise for different armies (as it used to be in the 6th ed.) sounds great.
For the army of Moot the situation should be vice-versa. Halfling units are core, human core units are special, human special units are rare. Human rare units are forbidden.
Then, halfling units might be like that:
-- Infantry w\bows
-- Infantry w\spears
-- Warmashine, Hot pot (may be special?), looking more like a catapult, rather than a slingshot
-- Halfling wizard (probably lores of life, beasts, metal only)
-- Halfling captain\lord
That would be an army of good fun