If you want to make enemies, try to change something.
There will actually be more mercs/slaves available to be used but they have different slot requirements depending on how strong they are (i believe i read that, so if you plan on having mercs that use good weapons they will cost more space than lets say those wielding spears.) They will also require upkeep for slaves/mercs so if someone actually has an army of them they have to pay for them more than once. I think that alone balances it out pretty well. Mercs are already expensive as hell adding paying them (daily weekly or whatever it is) will probably keep most people from using them and those that do have to earn them. So when you see a merc/slave army don't bitch about how it's unfair think about how hard it is to keep them.