Sazed also says, however, "All he would have to do was spend the occasional bit of time aged to give himself Feruchemical storages to burn and stay young."
And here I thought we were talking about something a little different with Feruchemy. You're storing some age, but simultaneously, you're burning that metalmind. That burning would give a net gain immediately. So in this potential Feruchemy-Allomancy trick, you're not really spending time aged; you immediately get that power back.
Of course, we do see that the Lord Ruler spends time aged, so I don't know that you can do this. For some reason. (Or Brandon hadn't thought of it.)
What happyman means is that it is through the Hemalurgic flaw in the kandra and koloss's minds which allowed the Lord Ruler to manipulate them in the first place.
And his over-powerfulness is, as stated by Sazed, something all the original ten Allomancers had.
I always assumed that the Lord Ruler was a brass savant, from all his Soothing. I don't think that's much of a stretch to say.