I really don't understand what you are asking for.
First you talk about using Eldar models for Black Legion (which would be ok if they were painted in BL colors and had some minor conversions) and then you talk about using chapter-specific colors on thousand sons (which you can, don't see why shouldn't).
Or do you mean that you want to play an basic, vanilla undivided chaos army with elite units that have a mark of chaos?
I am bit confused :wacko:
First you talk about using Eldar models for Black Legion (which would be ok if they were painted in BL colors and had some minor conversions) and then you talk about using chapter-specific colors on thousand sons (which you can, don't see why shouldn't).
Or do you mean that you want to play an basic, vanilla undivided chaos army with elite units that have a mark of chaos?
I am bit confused :wacko: