How is it overkill? If hes lets say having 2 scoops of concentrate and now hes having 1 scoop concentrate and 1 scoop isolate, its the same thing but better. I personally use only isolate. My diet does not allow for the extra carbs in concentrates, even though there are some with low carb, I know whats in my iso.
The best time to take it is anytime really. Although real food is better then whey, so only use it if your strapped and I wouldnt suggest replacing anymore than 2 meals out of 6 throughout the day, dont be lazy cook your food the night before.
He who dwells in the shelter of the Most High will rest in the shadow of the Almighty. I will say of the LORD, "He is my refuge and my fortress, my God, in whom I trust." Psalm 91:1-2