It's an interesting idea... Few things...
~The henchmen should include Human, Skaven, Beast, & Demon. This way, all four types can be represented, and your future heroes should come from either.
~With the above in mind, drop the Scum. The plague monks are only 20gc, you don't really NEED anything less. If you want to include lesser nurglings, include a spell or leader-only skill that lets you take 1D3 Nurglings like "Children of the Horned Rat".
~In reference to human henchmen, why not have "the Infected"? Similar to as zombies, except they can run and learn Exp. They are basically super slow humans that are so decayed that they feel no pain and are terrifying. With the running and exp added, somewhere around 25-30 gc sounds good.
~How does demonic instabliity work with Demonic heroes? Do they receive -1 or -2 to injury rolls?
Likewise, demonic heroes seem an odd mix. Normally, daemons occur as freak blips on the radar, as bound creatures, or the leaders of armies. The idea of having two in your warband as heroes without having them lead seems funny, that's all. Maybe tie them into a spell caster?