The general consensus (not only here, but almost everywhere) is that IO is adequate. My experience with it was completely different however. Now, I could have been unlucky and purchased a bad batch, but in any case, my calcium levels were terrible. I'm just adding that in for what it's worth.
As far as mixing goes...it's salt people. Salt plus salt does not yield bleach. Mixing is fine, but as far as preventing system shock, I don't believe it's necessary. That is, unless you are planning on doing a huge water change. If you have left over salt, you might as well mix it in as to not be wasteful and condition your system for it.