I dislike this idea intensely. While I agree and see where you are coming from on your points of balance/power and avoiding abuse across cult troops, I disagree with the idea that a special character designated as a cult special character should unlock the ability to field cult troops as Troops. I disagree because I don't think that this would be fluffy as you describe in some situations, Can we both agree that when we say Special Characters we are reffering to Ahriman, Typhus, Lucius and Kharn?
If so then i present my example based on that precident: Typhus unlocks the ability to take Death guard as Troops, a fine rule for anyne wanting to respresent the outward and detached style of warfare that typhus favours on a galactic scale - detached and distant from the core of his legion and his primarch. By taking Typhus it implies to me that I am fielding the Death guard of Typhus' fleet, which I don't, I field Death Guard of the great companies under the control of the Primarch himself, which has nothing to do with Typhus and his fanciful forays into Space. Another example I might use would be the Cabal of Ahriman, where Ahriman would unlock Thousand Sons as Troops - All well and fine for someone who want's to represent the Cabal of Ahriman on the tabletop. However the Cabal of Ahriman was expelled from the Thousand Sons Legion, and therefor act independently, and what of the people who would wish to represent an army under the command of one of the generals still in the favour of Magnus the Red?
I'm sure I have made my point clear with these two examples. I just feel that making the same decisions with Special Characters from the C:SM with Special Characters from C:CSM, is not a good direction to go in from a background standpoint, because the Legions are fractitious amongst themselves. To the players that use the armies of chaos I feel it is more about the individual Characters and their place in the setting, rather than the feel of the army as a whole - that unform feel that a Special Character of the Space Marines would give a player.
Basically, I play Death Guard, but I don't play Typhus' Death guard. Please do not force me to play with Typhus.
J