I'm not sure all of those would be problems. Marcus didn't necessarily use his power to get them into the Family. That would seem to be taking away from the other's capabilities and talent. They all had the capability to get in. The only thing Marcus did was recommend them, basically put up their names for consideration. As for his age as the leader, I looked up this RP and found out it was based on a visual novel/anime/something like that. When I researched the previous characters who were in positions of command, a lot of them were a lot younger than this, which is why I decided to follow that norm and also make my character a younger leader. The previous Head of Sword, according to the actual thing, was only 17 or so.
And about the working solo category, Marcus does have a preference to work alone, but that isn't exactly common knowledge. As I've also written in his SU, he keeps his personal opinions out of his work, including this. I also did say that despite his preference he does work well with others in teams, and enjoys being in positions of command.
So I still think it shouldn't be too much of a problem. At the very least they'd probably be well-known acquaintances, in my opinion.