I do think that some dogs, especially the males, actually enjoy being not-the-boss. They don't think, "Geeze, I keep getting henpecked by these girls, this is rough!" or "Aww, I never get the frisbee like I did when I was by myself". They seem to think, "Hooray, I don't have to make the decisions anymore!" or "Man oh man, I love having these girls around!"
When we brought Mark home, he was the only male at our house, and he was immediately made low man on the totem pole... here was this young, zippy, little-bit-macho intact male, and the girls basically saw him as theirs to beat up on. They'd shove him around, chase him down, flip him over, and generally make a fool of him at every chance. And honestly (and I know my dog), he
loved it. Those girls told him where to go and what to do, and he'd say "yes ma'am!" He was always a very confident, happy dog (and a successful stud dog), so I don't think he felt "bad" about it in the least. It's a dog thing.
What I'm trying to get at is that I think that a puppy for Sca sounds like a really great idea. As much as you adore him, and as well as you care for him, there are certain nuanced social interactions that he can only get with a canine buddy. Even if he does end up being bossed by the new puppy, he'll be a-OK with it (and if/when he isn't, he'll make a nasty face and some loud noises and the puppy will learn not to push that particular button). I say go for it.