Yes, I like it too! I don't know about the kind of bonus-malus, probably both morale or CS would do the work, maybe the easier to code? 
To 'get' Leadership you can battle... or train it in your academy! I know, an exotic concept! 
This is only a rough idea:
A Marshal with 5% Leadership could command 1 unit (basically, his own!) for each 5% he would command 1 unit more... with a 50% Leadership a Marshal could command an army of 10 units. Bonus/Malus, for every unit over the max. it will take a -5%, for every 2 units below, it would get a +5%?
A High Marshal will command 1 army from 0% to 25%, 2 armies from 26% to 50%, 3 from 51% to 70%, 4 from 71% to 80% and 5 from 81% to 100%... bonus and malus?... maybe for every army over his limit a -5%, and every army below, a +5%? ... if there is no High Marshal the limit would be zero.
I that its a little low on the marshals one. Instead of High Marshal while we are discussing for simplicity could you use the title general? Your percentages sound good though with the general one.
