Mixing giants to kings is OK - IMO they are basically the same, the difference being giants have pure giant genes while kings do not. Color wise too isn't a problem. The male should carry a few hidden genes and the female is a multi - the more colors the better.
I'd breed them - keep the mother in a rather small container to stunt her growth and boost fry's growth. Take the best (to your preference) fry and breed back to mom. If you let mom grow further, fry might not be able to out grow her in time.
As for aggressiveness - I prefer them aggressive. Non aggressive female might hide more which will make aggressive males attack .... more risky. Aggressive female usually spawns faster. Just make sure that she is in breeding mode when you release her - older/bigger giants females are often harder to breed and males can be more deadly.
Last edited by indjo; 09-29-2012 at 08:05 PM.