This wouldn't be a 'merging' of units (especially considering that Hama's guard is limited to one unit with a duration timer attached), it would simply make an existing unit much more powerful by giving it a very strong new option. And while I think this might actually be a pretty good idea for the Royal Guard, it's not a very good reason to double their recruitment limit on top of that. As others have said, Rohan is not meant to be a faction that can field six heroic infantry units.
As for comparison to other factions, Mordor actually only gets their full seven heroic units once you control two outposts and two very expensive buildings, that's quite a difference from simply getting six heroic units in your basic stables. Also note that Mordor has no upgrades and very few elite units. Gondor has more than Rohan, Isengard has less, and the Dwarves have potentially more but tied to a more complicated recruitment scheme that also needs a settlement and can this be disrupted. Lorien will also have only one heroic unit, so I don't Rohan is unfairly behind here.
I'd still like to try out a mount for the Royal Guard, but not with an increased limit.