I don't see the issue though. Even the way you go about it is 'in character', as you go to a tavern to 'hire' a mercenary (yeah, you have to pay for them). So create one if you're finding you need to round your party out, then just drop him off at your stronghold when you find another companion that you want to keep in the party. There's no effect on the game, other than the XP effect (slightly less XPs per character with bigger parties), but you don't strike me as someone that worries about that - and you have to deal with that when you add NPCs to your party anyway.
Sorry, not trying to be argumentative, I just wanted to point out that it's a viable option if you're having troubles with the combat difficulty, and there's no real reason to not do it if needed.