Ability to hire any sort of mercenaries should be in the game for diplomatic characters - BUT - only when the story and narrative would make it seem reasonable.
Again, instead of going for dumb crude extreme design decisions such as allowing something everywhere in every case or not allowing it at all, one should design such options adhering to internal logic and coherence of the game story.
The notion that diplomacy path should be made with only dialogue options that never result in any fight is complete convoluted unrealistic fantasy notion.
Thats not what "diplomacy" is limited to.
As you can see in the game itself, you are already making various deals and affecting other factions in bigger ways, often ending in those NPCs getting killed, or whole factions destroyed.
Instead of simply hiring mercenaries left and right, which would be completely stupid idea, the player who goes down one of the diplomatic paths should be able to influence factions in the city so they do some things for him, much as the examples of bandits and thieves guild already shows.
Some smaller such events could be made - guided by the narrative itself - where the player talks one of the factions into "jumping in" into some problem.
No need to allow that for every quest and situation, instead just choose those where it would fit.
- you can constrain these options though internal narrative reasons, lack of money, and branching gameplay that closes different routes depending on choices - which is all already part of the game.
Thieves and Assassins wont simply do any job you have for them, nor could you pay for their services with just money - since other "political" matters have to be taken in consideration.
You could hire thugs to do something for you but that should not result in the best outcomes.
Plenty of stuff to play with there.
It only becomes unsound, incoherent and stupid is if its done in some extreme way where such options are allowed all the time. And in a way that would turn the game into a party based game.
Which would be completely unfitting for the setting as it is, and for the narrative of the game as it is.
You should be able to make such deals from time to time, with varying consequences. Most of such deals should be resolved off screen, (just like bandits attacking the mines, or thieves attacking the bandits does) and in a few cases where you would be a part of such a fight you should never be able to control the "mercenaries" yourself.
The only problem with this approach is that it would require more time spent on making the game, not that its unsound in any way.
Since this game wants to be both difficult and as non-metagame as possible,
Its one of the most meta gaming games - ever.