I'm not crazy about the grading thing either really, and honestly I've seen many B's I liked much better than some A's.
Personally I just like the random patterns whatever the grade may be, everyone has their own idea of what looks cool, I pretty much like em all.
Yes you can mix snow's and percs, make sure at least one is no bigger than an inch and a half, preferably smaller if you can, and I advise intro in a specimen container to be safe.
The one w/ a tattered dorsal also has a ragged tail, probably has been bullied.
It could heal, but I do prefer starting w/ clean healthy as can be clowns, and they should be QT'd to be safe.