So the Tangs are the ones that use the shells and need a higher pH, so are they only for salt water?
some of the points you pointed out are partially true Rok. The pH is pretty close in both lakes. I would say overall Lake Malawi species are more aggressive...but don't rule out all tangs, as some can get nasty when they need to. Lake Malawi Cichlids don't really "school" per say....they just kinda swim frantically around their territory and make it look like a school. Tangs seem to be slower, and would rather dwell by the rocks or just kinda float along, at least in my experiences.
Mixing lakes can work, you just need the right stocking plan and diet plan. I've kept Tangs and Malawi together with relative success.