Mystery snail is something of a marketing label. Generally, a mystery snail is simply an apple snail.
If you are worried about your krib being aggressive with the snails, you might consider a snail with a smaller profile, such as a nerite snail, though they are not so easy to find. They will not successfully breed in freshwater, and they are much better at cleaning the tank than apple snails.
I have never seen my krib even take notice of my snails, but each fish is different. My krib is pretty calm, but yours might not be. For instance, I've kept a
betta that lived peacefully with 20 or more fish sharing his living space, and I currently have a betta that will only tolerate snails in his environment--anything else better be prepared to duke it out.