Empress & Queens are Managers
While Aries can be leaders, most Astrology forums I go to agree Leo would be the one generally associated as "leader" (hence they're considered the best organizers after Virgo and the one with the best managerial skills)
Ah. This isn't as much of a head-scratcher as you think. You see, in Tarot, the Queens (and Empress) often have an equal/opposite relationship and in certain decks, they are the "Leaders" when you're talking managerial skills. The Queens are usually the fixed signs. Meaning the ones who run the show while the King is out there being the politician, the preacher, the firebrand, the idea man, the great general. So if you have a King/Wands (Aries) then your Queen/Wands is Leo.
So, yes, Leo is the leader if you're talking about hands-on management.
Now you're getting into a whole other topic altogether >.> when you started trying to bring up Mars as the planet of Scorpio.
Tarot was created and astrological associations given to it long before Pluto was discovered. When Pluto came onto the scene newer decks incorporated it. I've no idea what Astrologers are doing now that Pluto has been demoted and we're officially at eight planets rather than nine.
Still Aries as "DISCIPLINED" and "STOIC"?
Not from what I've read and learned. Quite the opposite actually. Aries/Emperor is often viewed as the "baby"--who is Emperor of every household. Everyone is his servant. If he's a good baby, then he's irresistible and everyone wants to do things for him. But if he's a bad baby...Remember--all tarot cards can be ill-dignfied and have negatives, and the big negative of the Emperor is this bad-baby side that is always demanding something and has no patience, discipline or stoicism.
His value is in seeing possibilities that others can't see and communicating those to them in a way that gets everyone moving in the same direction, be they warriors or inventors or builders. THAT is the kind of leader he is--but that is not a leader who is going to be patient or stoical. That's a man who can't understand why everyone can't see what he's seeing and just do what he says, because he knows how good it will be if they obey him. That's not a hands-on-manager.
Which is where the Empress steps in--she is Venus (think Taurus-esque)--and she is in every way his equal, his partner, and a leader in her own right. It is her job to cultivate and grow things, to be the ultimate Mother, protecting what is fragile and needs nurturing till it can leave the nest--be that children or new-idea-men. She is slow and patient and insists on giving things as much time as they need; she brings beauty and civility, arts and stability to the Empire. What the Emperor lacks, she has and vice versa. She is the hands-on-manager.