Troop compatability.

[Ryastar]

And anything that doesn't have a race, ie. they are neutral, gain no morale bonuses or penalties from all other troops in your army, and do not affect the morale of all other troops.

And all this is unless there is some creature specific ability that will affect its morale. For example, inquisitors are holy so they do not lose morale from having undead in your army even though they are a human troop.