Technical implementation.

1. The number of units is determined randomly, based on certain criteria (morale, the number of units the hero has, the size of their army, the type of creatures, and others).

The number of units depends on the ratio between the hero's army strength and the neutral army's strength:
1. Neutral army strength = Creature strength * Number of creatures
2. Hero's army strength = Sum of unit strengths (Creature strength * Number of creatures * Luck coefficient * Morale coefficient * Artifacts initiative coefficient).
The only unknown is the number of creatures in the neutral army. If there is scouting, then everything is fine; if not, you have to take a range of creatures.

2. Silent Hunter already allows you to find out the creatures' disposition...
therefore, I don't see the point of such a mod (no offense).

Silent Hunter allows you to find out whether they will always fight (ALWAYS_FIGHT), always join (ALWAYS_JOIN), or their decision is undefined (CAN_FLEE_JOIN).
In the latter case, they may join, or they may want to fight. I want to know the chance of them joining.