CO2 will kill pinkies...eventually. But it takes MUCH longer to kill pinks with CO2 than even fuzzies. If you just freeze them, they will huddle together before they die. It takes about 1 1/2 to 2 hours to freeze them. I know that some people find freezing the pinks to death to be inhumane. But it is the best way I have found to do it, especially in numbers. What I do is put the live pinks in a plastic container and put that in the freezer. After about 3 hours (it would be REALLY cruel in my oppinion to put them in there and then take them out before they are dead) I take them out, separate them, and then vacuum seal them individually, or in small "dinner packs" of about six. Then I freeze those packs. The first "freeze" is just to kill them. They aren't really stuck together too bad after that point, so they aren't hard to separate. If you leave them in a bunch overnight, that starts getting trickier. Also, since I vacuum seal them, I have found that it is better that they be a little frozen first. They are, well...sturdier when mostly frozen. There have been people who ask "Why not just vacuum seal them while they are alive? If you take out all of the air, they'll die." My answer is, it's messy that way. They tend to "squash" if they aren't at least partially frozen first.
So...(I'm sorry for the long-winded answer) Yes freeze them dead, but not solid...then separate them, and freeze them solid.