I thought that the monster behavior was determined at the start of the game and doesn't change during the game: "At the moment the game is first launched, the probability of a unit joining is randomly determined." (PhysMiG). My army remained the same (minus one stack)!
In another case ("Island Edge"), phoenixes wanted to join, and I decided to recruit them with another hero of the same faction, with creatures of the same faction – the phoenixes attacked!
(Василь Симоненко. Люди – прекрасні)