yes, it's perfectly safe.
nickel tends to work harden, and wear out quicker in my experience, so i prefer brass. however, 45 is much more forgiving in this manner, as opposed to 9mm, for instance, and i can still get several loadings out of one 45 nickel case before it shows signs of extreme fatigue.
that said, i've never had a nickel case outlast a brass one, and i'm a little OCD, so if i pick up brass and find only one or two nickel cases, i dont' mix them with the rest. not from any safety standpoint... purely cosmetic OCD. :P
the choice to reload 45 is a good one. it's very forgiving in almost every way. good place to start.