Actually, I find it's a pretty good idea !
There was a lot of work - hard work, to create the three different kingdoms. They have very specific things (crossbow/bow/axe thrower ; elite units, different heroes..), unique graphism too, but why not more diversity/units with this ram idea ? Even adventurers would be a nice addition !
For Erebor and Khazad Dum I don't know, of course it came from Erebor but it's a central position that can help the three kingdoms. And it's good to have a "transversal" units in the three factions.
Another specific unit for Erebor would be great. Royal guard or... I don't know what.
But it's a good idea that the three dwarves kingdoms have specific gameplay too, if one of them had ram for mobility (but not for every dwarve kingdoms, they are mainly slow !).
More differences between kingdoms would be great, like elves with Elrond/Galadriel/Thranduil differences (after I'm not a specialist of Tolkien and I don't know if that's possible, dwarves seem closer between them than elves).