Already done. They have caps that make a stack have 9 Cohorts and 1 First Cohorts.
The problem is that in the engine there's not an army cap for campaign mode. The only cap you can give is total, i.e. for example for a cap of 5 you can give a total cap of 75 (5 * 15, 15 being the maximum number of stacks available).
So for Romans the cap is 135 for Cohorts and 15 for First Cohorts, in total 150 that is 10 per stack in a balanced faction. Still the user can decide to have a stack almost full of First Cohorts if he so decides, given this.
Sadly there's nothing to do about that. If I could give caps per army as in MP I would have done that instead. There are some things being done by CA that I really cannot understand. I don't understand what's the point of giving a proper cap per army in MP but not give the same option for campaign mode.












