I suggest you ask Greenleaf Nutrients about this. Having diluted nutes sit for a limited time mixed in a reservoir could be much different chemical stability-wise vs, long-term storage of nutes in concentrated solution. Keep in mind some nutes may cross-react with others, with this presumably the reason most liquid nutes come as multiple parts to be mixed.
Pre-mixing Greenleaf Nutrient's powdered nutes should not be a problem. Why not just weigh-out pre-mixed GN nutes as powders, then just add them to water?