I think I've found a solution: just set a repeating event for the peasants: every day, all peasants will be expelled, and 10 will be added.
However, I haven't tested this method.
If it doesn't work, you can try this:
1. Give 1 leprechaun.
2. Expel all peasants.
3. Give 10 peasants.
4. Expel the leprechaun.
This should work 100%.
My method also works 100%, and it doesn't matter how you limit the creatures, the main thing is that it works.