Ally - anything aiding you in your objective

Hostile / enemy - anything attempting to impede you in your objective

Civilian / by-stander / neutral - anything that doesnt apply to ally or hostile.

Basically, if it helps, it can be considered an ally, even if its normally an enemy. An example of this is Darth Vader at the end of Jedi.

In your question I would simply have the character use their skill checks to see if they could train the mice effectively. We have used animals on war for all kinds of things from messengers to weapons. I dont see a reason this person couldnt use mice.

Ally - anything aiding you in your objective

Hostile / enemy - anything attempting to impede you in your objective

Civilian / by-stander / neutral - anything that doesnt apply to ally or hostile.

Basically, if it helps, it can be considered an ally, even if its normally an enemy. An example of this is Darth Vader at the end of Jedi.

In your question I would simply have the character use their skill checks to see if they could train the mice effectively. We have used animals in war for all kinds of things from messengers to weapons. I don't see a reason this person couldn't use mice.