Ok, first let me say- I want everyone to choose and play the characters they feel most connected and comfortable with.
Now, for the second, son- if it helps at all, I think that a man genetically engineered specifically for a military career should find it extremely easier to develop muscle mass, among other physical characteristics you'd need to excel through the military ranks (probably not a grunt anymore, he can be anywhere from a military officer to the FBI/CIA, but it would probably be a station that keeps him close to home when the game starts).
As for the rest of them, I don't want to step on any toes, but when I've picked my face-claim, for the brother with the actual 'family-mattress' role, I did my best to choose one that would be seductive enough to serve as a bottom, but still manly enough to allow him to fulfill the role of a top whenever he needs to? Does that sound reasonable?
We haven't talked about top/bottom/dom/sub roles yet, but I'd like you to take two points into consideration-
1. Each brother needs to be and look older than his younger ones by at least a year and a half (to allow for the genetic treatments, seeding, cultivating and embryo selection).
2. Even in today's society, male power and dominance are often associated with physical superiority.
I hope I haven't offended anyone's sensibilities here, I really do want you to play the characters that you want to play, just felt like it needed to be said, for the plot's integrity if for nothing else.