1. its likely that they will both run at 1600mhz cl10 at 1.5v. Since when u mixing ram,the weaker one serve as reference.
2. No, dont mix them ,save u the trouble. Sometimes, even 2 pairs of exact model ram will not play together.(Quote from Tradersman) not to mention 2 difference brand.
3. I highly think that 8gb 1866 cl9 will have better performance than 24gb 1600 cl10