So this might have already been made, but I just randomly thought about it and wanted to see if maybe it might be a cool challenge? idk
It's pretty self-explanatory, from the name. There are three modes, easy, medium, and hard.
Easy: Baby-1 day Child-1 day Teen-2 days Adult- 10 days Pregnancy duration- 1 day
Medium: Baby-1 day Child-1 day Teen-1 day Adult-7 days Pregnancy duration- 2 days
Hard: Baby-1 day Child-1 day Teen-1 day Adult-4 days Pregnancy duration- 2 days
Basically, your nichelings have limited lifespans, making it more difficult for them to live long enough to reproduce and pass on their genes. Other than that, basically do whatever you want!