I think Scout will be just fine, and it might actually help him relax a little more if he sees the other kitten relaxed in his surroundings. And if you are going to get the kitten at 6 weeks, it would be good for it to have Scout around to help ease him away from his mom and sibs. Much easier to have Scout around then to be thrown into an environment with no other critters at all, then have to spend a lot of time alone while your BF works.

I really think the hardest part will be you having to give her up when the time comes. Also, then Scout might get used to having another kitty around and be heart broken when she leaves, then you'll just have to get another - oh how terrible that would be.