I will say no do not mix, For reloading throw out the nickel brass and use uncoated Win brass. The nickel will start flaking later in life and can cause damage to the reloading dies. (I used the nickel in the past beacuse i liked the look, call me stupid!)
If i were you, go buy double what you want for finished cases, run them through the expander ball to make all the necks round, then trim all to the same lenght, and lastely weight sort them.
The only problem i have had with win brass is consetency between bags. Get a buch up front and shoot them tell they are done! then start over.
Some my disagree with the weight sorting, and if you do, just go by a bag load it and shoot. If you have a good load worked up and you get a flyer mark the brass. if it dose it again on the next reload toss it.
Willys