I certainly wouldnt loose any sleep about mixing anti freeze however if you are going to add some its a good idea to drain the system and get rid of the old coolant and start again with fresh antifreeze/water.
But if you just want to avoid freezing as its winter drain out a litre and add a litre of a/f
And electrolosis isnt something you add its just a reaction between the metals in your engine which eats away at water jackets and the galleries/fitting etc causing headgasket failure usually.
Antifreeze also has anti corrosion which prevents this happening making the anti corrsion properties more important in my opinion than the antifreezing side of things unless you cruise antartica on a regular basis.