Your questions boil down to:
- What is Mass Combat Leadership and do you have to have the Leadership feat.
- If Leadership feat is not needed how does the feat effect Mass Combat.
- Will taking Leadership early make my character overpowered and should I offer one of my abilities in return for getting the feat early.
So lets answer these questions.
- Mass Combat leadership does not require the Leadership feat. (See quote below)
- The leadership feat does effect mass combat's leadership score if you are the commander of an army.
Leadership: The commander's character level + Charisma modifier. If the commander is a monster, use HD instead of character level. If the commander has the Leadership feat, increase this value by 3. This number is a prerequisite for some boons.
Just to clarify at no point other than the above qoute does the Mass Combat rules reference the Leadership FEAT's score. Only the above score. If it helps think of all references to Leadership in Mass Combat as Commander Score
- Leadership is not broken as the DM controls every aspect of it. Your Cohort and followers are all controlled by the DM. Hey I can Chan covers this in his answer quite well.
First lets look at how leadership the Feat is calculated.
Leadership Score: Your base Leadership score equals your level plus your Charisma modifier.
You are level 3 and your CHA Mod is 4. So your score is 7. Looking at the leadership chart you would have no followers at all and your cohort would be limited to level 5. But since your Cohort has to be 2 levels below your level he/she would be level 1.
This wouldn't really break your character significantly as until you reach level 6 you wouldn't have any followers, just your cohort. At level 6 you would gain 5 level 1 followers. At that point they are mostly useless in battle. So for levels 3-5 you would just have your cohort who won't effect the party in anyway your DM shouldn't be able to handle easily.
Now that leadership the feat is covered lets look at mass combat and how its most important stat Army Challenge Rating(ACR) is calculated and if leadership would effect that in a significant way.
An "army" of one is a fine army size and its ACR is -8 of the individual creature's CR. At level 3 your CR is 3 and even with your cohort you don't meet the prerequisite of 10 for the next army size up.(Which still takes a -6) Until the army gets up to 100 creatures in size it will be taking negatives to it's ACR. And once it is 100 it is the same level as most of the army. So an army of level 1s would still only be ACR 1.
ACR is highly significant in Mass combat because it determines Defense Value(Equivalent of AC) and Offense Modifier(BAB) so a negative ACR means you have a very small chance to hit a force larger than yourself.
ACR also determines the armies health.
So Leadership is not broken even if taken at lower levels. When used to strengthen your army it doesn't really effect anything at the level you are at. Even at higher levels it wouldn't help much.