I do not know what a roo is. It sounds like you are referring to cockerels. LOL. I am no seasoned breeder. I do not know what "pretty aggressive" is either. That could mean anything from vigorous to a man fighter. If the later, chop his head off. They are not worth messing with. Regardless of all of the amateur theories (I have heard a lot and smiled), and 'behavior modifications", the bird would have a few "screws loose", and the characteristic is highly heritable. Breeding one is producing more like him. Really, I do not see where it matters (unless the one is a man fighter). It sounds like an experimental cross to make layers of olive colored eggs. The requirement is a bird from a good dark egg laying strain x a good blue egg laying strain. There is no standard for type or color except your own. You cannot visually tell which would produce better layers that laid bigger or more eggs. To know that would be to prove them by their offspring. If you are considering trying to breed cuckoo Marans also (which is not clear), then good Marans type should be your goal. Cuckoo is not a difficult color, and could be worked on along the way. You are the one having to deal with them, and it sounds like you will enjoy the second bird the most.