I have an important question, can undead and humans be in the same squad of the hero? - Of course they cannot, the problem is that the general morale of the detachment (people) will go down. Morality doesn't work for undead. If a squad had low morale in HoM3, then it would skip a turn. And we have nothing like this, although the hero may well hire two (incompatible in outlook) armies without any drawbacks.
2. If the hero's squad consists of creatures from one castle, then their morale must be +2. And for each creature of a different worldview, shoot -1 morale. The undead will always have 0. And in order to eliminate many debuffs, the player will have to pump leadership.
Morale -1 = -10% attack / life / speed
Morale -2 / -3 / -4 / -5 = -15 / 20/25/30%