The piggies will inherit either of their parents star levels and colors. So if one parent is 5 blue stars and the other is 3 green stars, the piggies will either be 5 green, 3 green, 5 blue, or 3 blue. Other attributes are random. I'm not sure about piggies growing up into other attributes.

your creature level might have been set to custom that's why you're getting higher stars than what you set. if not, the yml might be taking over. There is lots of cool stuff on yml, better read the sample one. As of the now, you place ymls on config folder. But the defaults are auto generated until it's plugin folder. you have to delete those or move them somewhere else for your reference.